src/HOL/Tools/sat_solver.ML

 Interface to external SAT solvers, and (simple) built-in SAT solvers.
 *)
 
 signature SAT_SOLVER =
 sig
-	exception NOT_CONFIGURED
-
-	type assignment = int -> bool option
-	type proof      = int list Inttab.table * int
-	datatype result = SATISFIABLE of assignment
-	                | UNSATISFIABLE of proof option
-	                | UNKNOWN
-	type solver     = PropLogic.prop_formula -> result
-
-	(* auxiliary functions to create external SAT solvers *)
-	val write_dimacs_cnf_file : Path.T -> PropLogic.prop_formula -> unit
-	val write_dimacs_sat_file : Path.T -> PropLogic.prop_formula -> unit
-	val read_std_result_file  : Path.T -> string * string * string -> result
-	val make_external_solver  : string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver
-
-	val read_dimacs_cnf_file : Path.T -> PropLogic.prop_formula
-
-	(* generic solver interface *)
-	val solvers       : (string * solver) list ref
-	val add_solver    : string * solver -> unit
-	val invoke_solver : string -> solver  (* exception Option *)
+  exception NOT_CONFIGURED
+
+  type assignment = int -> bool option
+  type proof      = int list Inttab.table * int
+  datatype result = SATISFIABLE of assignment
+                  | UNSATISFIABLE of proof option
+                  | UNKNOWN
+  type solver     = PropLogic.prop_formula -> result
+
+  (* auxiliary functions to create external SAT solvers *)
+  val write_dimacs_cnf_file : Path.T -> PropLogic.prop_formula -> unit
+  val write_dimacs_sat_file : Path.T -> PropLogic.prop_formula -> unit
+  val read_std_result_file  : Path.T -> string * string * string -> result
+  val make_external_solver  : string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver
+
+  val read_dimacs_cnf_file : Path.T -> PropLogic.prop_formula
+
+  (* generic solver interface *)
+  val solvers       : (string * solver) list ref
+  val add_solver    : string * solver -> unit
+  val invoke_solver : string -> solver  (* exception Option *)
 end;
 
 structure SatSolver : SAT_SOLVER =
 struct
 
-	open PropLogic;
+  open PropLogic;
 
 (* ------------------------------------------------------------------------- *)
 (* should be raised by an external SAT solver to indicate that the solver is *)
 (* not configured properly                                                   *)
 (* ------------------------------------------------------------------------- *)
 
-	exception NOT_CONFIGURED;
+  exception NOT_CONFIGURED;
 
 (* ------------------------------------------------------------------------- *)
 (* type of partial (satisfying) assignments: 'a i = NONE' means that 'a' is  *)
 (*      a satisfying assignment regardless of the value of variable 'i'      *)
 (* ------------------------------------------------------------------------- *)
 
-	type assignment = int -> bool option;
+  type assignment = int -> bool option;
 
 (* ------------------------------------------------------------------------- *)
 (* a proof of unsatisfiability, to be interpreted as follows: each integer   *)
 (*      is a clause ID, each list 'xs' stored under the key 'x' in the table *)
 (*      contains the IDs of clauses that must be resolved (in the given      *)

 (*      The clauses of the original problem passed to the SAT solver have    *)
 (*      consecutive IDs starting with 0.  Clause IDs must be non-negative,   *)
 (*      but do not need to be consecutive.                                   *)
 (* ------------------------------------------------------------------------- *)
 
-	type proof = int list Inttab.table * int;
+  type proof = int list Inttab.table * int;
 
 (* ------------------------------------------------------------------------- *)
 (* return type of SAT solvers: if the result is 'SATISFIABLE', a satisfying  *)
 (*      assignment must be returned as well; if the result is                *)
 (*      'UNSATISFIABLE', a proof of unsatisfiability may be returned         *)
 (* ------------------------------------------------------------------------- *)
 
-	datatype result = SATISFIABLE of assignment
-	                | UNSATISFIABLE of proof option
-	                | UNKNOWN;
+  datatype result = SATISFIABLE of assignment
+                  | UNSATISFIABLE of proof option
+                  | UNKNOWN;
 
 (* ------------------------------------------------------------------------- *)
 (* type of SAT solvers: given a propositional formula, a satisfying          *)
 (*      assignment may be returned                                           *)
 (* ------------------------------------------------------------------------- *)
 
-	type solver = prop_formula -> result;
+  type solver = prop_formula -> result;
 
 (* ------------------------------------------------------------------------- *)
 (* write_dimacs_cnf_file: serializes a formula 'fm' of propositional logic   *)
 (*      to a file in DIMACS CNF format (see "Satisfiability Suggested        *)
 (*      Format", May 8 1993, Section 2.1)                                    *)
 (* Note: 'fm' must not contain a variable index less than 1.                 *)
 (* Note: 'fm' must be given in CNF.                                          *)
 (* ------------------------------------------------------------------------- *)
 
-	(* Path.T -> prop_formula -> unit *)
-
-	fun write_dimacs_cnf_file path fm =
-	let
-		(* prop_formula -> prop_formula *)
-		fun cnf_True_False_elim True =
-			Or (BoolVar 1, Not (BoolVar 1))
-		  | cnf_True_False_elim False =
-			And (BoolVar 1, Not (BoolVar 1))
-		  | cnf_True_False_elim fm =
-			fm  (* since 'fm' is in CNF, either 'fm'='True'/'False', or 'fm' does not contain 'True'/'False' at all *)
-		(* prop_formula -> int *)
-		fun cnf_number_of_clauses (And (fm1,fm2)) =
-			(cnf_number_of_clauses fm1) + (cnf_number_of_clauses fm2)
-		  | cnf_number_of_clauses _ =
-			1
-		(* prop_formula -> string list *)
-		fun cnf_string fm =
-		let
-			(* prop_formula -> string list -> string list *)
-			fun cnf_string_acc True acc =
-				error "formula is not in CNF"
-			  | cnf_string_acc False acc =
-				error "formula is not in CNF"
-			  | cnf_string_acc (BoolVar i) acc =
-				(assert (i>=1) "formula contains a variable index less than 1";
-				string_of_int i :: acc)
-			  | cnf_string_acc (Not (BoolVar i)) acc =
-				"-" :: cnf_string_acc (BoolVar i) acc
-			  | cnf_string_acc (Not _) acc =
-				error "formula is not in CNF"
-			  | cnf_string_acc (Or (fm1,fm2)) acc =
-				cnf_string_acc fm1 (" " :: cnf_string_acc fm2 acc)
-			  | cnf_string_acc (And (fm1,fm2)) acc =
-				cnf_string_acc fm1 (" 0\n" :: cnf_string_acc fm2 acc)
-		in
-			cnf_string_acc fm []
-		end
-		val fm'               = cnf_True_False_elim fm
-		val number_of_vars    = maxidx fm'
-		val number_of_clauses = cnf_number_of_clauses fm'
-	in
-		File.write path
-			("c This file was generated by SatSolver.write_dimacs_cnf_file\n" ^
-			 "p cnf " ^ string_of_int number_of_vars ^ " " ^ string_of_int number_of_clauses ^ "\n");
-		File.append_list path
-			(cnf_string fm');
-		File.append path
-			" 0\n"
-	end;
+  (* Path.T -> prop_formula -> unit *)
+
+  fun write_dimacs_cnf_file path fm =
+  let
+    (* prop_formula -> prop_formula *)
+    fun cnf_True_False_elim True =
+      Or (BoolVar 1, Not (BoolVar 1))
+      | cnf_True_False_elim False =
+      And (BoolVar 1, Not (BoolVar 1))
+      | cnf_True_False_elim fm =
+      fm  (* since 'fm' is in CNF, either 'fm'='True'/'False', or 'fm' does not contain 'True'/'False' at all *)
+    (* prop_formula -> int *)
+    fun cnf_number_of_clauses (And (fm1,fm2)) =
+      (cnf_number_of_clauses fm1) + (cnf_number_of_clauses fm2)
+      | cnf_number_of_clauses _ =
+      1
+    (* prop_formula -> string list *)
+    fun cnf_string fm =
+    let
+      (* prop_formula -> string list -> string list *)
+      fun cnf_string_acc True acc =
+        error "formula is not in CNF"
+        | cnf_string_acc False acc =
+        error "formula is not in CNF"
+        | cnf_string_acc (BoolVar i) acc =
+        (i>=1 orelse error "formula contains a variable index less than 1";
+        string_of_int i :: acc)
+        | cnf_string_acc (Not (BoolVar i)) acc =
+        "-" :: cnf_string_acc (BoolVar i) acc
+        | cnf_string_acc (Not _) acc =
+        error "formula is not in CNF"
+        | cnf_string_acc (Or (fm1,fm2)) acc =
+        cnf_string_acc fm1 (" " :: cnf_string_acc fm2 acc)
+        | cnf_string_acc (And (fm1,fm2)) acc =
+        cnf_string_acc fm1 (" 0\n" :: cnf_string_acc fm2 acc)
+    in
+      cnf_string_acc fm []
+    end
+    val fm'               = cnf_True_False_elim fm
+    val number_of_vars    = maxidx fm'
+    val number_of_clauses = cnf_number_of_clauses fm'
+  in
+    File.write path
+      ("c This file was generated by SatSolver.write_dimacs_cnf_file\n" ^
+       "p cnf " ^ string_of_int number_of_vars ^ " " ^ string_of_int number_of_clauses ^ "\n");
+    File.append_list path
+      (cnf_string fm');
+    File.append path
+      " 0\n"
+  end;
 
 (* ------------------------------------------------------------------------- *)
 (* write_dimacs_sat_file: serializes a formula 'fm' of propositional logic   *)
 (*      to a file in DIMACS SAT format (see "Satisfiability Suggested        *)
 (*      Format", May 8 1993, Section 2.2)                                    *)
 (* Note: 'fm' must not contain a variable index less than 1.                 *)
 (* ------------------------------------------------------------------------- *)
 
-	(* Path.T -> prop_formula -> unit *)
-
-	fun write_dimacs_sat_file path fm =
-	let
-		(* prop_formula -> string list *)
-		fun sat_string fm =
-		let
-			(* prop_formula -> string list -> string list *)
-			fun sat_string_acc True acc =
-				"*()" :: acc
-			  | sat_string_acc False acc =
-				"+()" :: acc
-			  | sat_string_acc (BoolVar i) acc =
-				(assert (i>=1) "formula contains a variable index less than 1";
-				string_of_int i :: acc)
-			  | sat_string_acc (Not (BoolVar i)) acc =
-				"-" :: sat_string_acc (BoolVar i) acc
-			  | sat_string_acc (Not fm) acc =
-				"-(" :: sat_string_acc fm (")" :: acc)
-			  | sat_string_acc (Or (fm1,fm2)) acc =
-				"+(" :: sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 (")" :: acc))
-			  | sat_string_acc (And (fm1,fm2)) acc =
-				"*(" :: sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 (")" :: acc))
-			(* optimization to make use of n-ary disjunction/conjunction *)
-			(* prop_formula -> string list -> string list *)
-			and sat_string_acc_or (Or (fm1,fm2)) acc =
-				sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 acc)
-			  | sat_string_acc_or fm acc =
-				sat_string_acc fm acc
-			(* prop_formula -> string list -> string list *)
-			and sat_string_acc_and (And (fm1,fm2)) acc =
-				sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 acc)
-			  | sat_string_acc_and fm acc =
-				sat_string_acc fm acc
-		in
-			sat_string_acc fm []
-		end
-		val number_of_vars = Int.max (maxidx fm, 1)
-	in
-		File.write path
-			("c This file was generated by SatSolver.write_dimacs_sat_file\n" ^
-			 "p sat " ^ string_of_int number_of_vars ^ "\n" ^
-			 "(");
-		File.append_list path
-			(sat_string fm);
-		File.append path
-			")\n"
-	end;
+  (* Path.T -> prop_formula -> unit *)
+
+  fun write_dimacs_sat_file path fm =
+  let
+    (* prop_formula -> string list *)
+    fun sat_string fm =
+    let
+      (* prop_formula -> string list -> string list *)
+      fun sat_string_acc True acc =
+        "*()" :: acc
+        | sat_string_acc False acc =
+        "+()" :: acc
+        | sat_string_acc (BoolVar i) acc =
+        (i>=1 orelse error "formula contains a variable index less than 1";
+        string_of_int i :: acc)
+        | sat_string_acc (Not (BoolVar i)) acc =
+        "-" :: sat_string_acc (BoolVar i) acc
+        | sat_string_acc (Not fm) acc =
+        "-(" :: sat_string_acc fm (")" :: acc)
+        | sat_string_acc (Or (fm1,fm2)) acc =
+        "+(" :: sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 (")" :: acc))
+        | sat_string_acc (And (fm1,fm2)) acc =
+        "*(" :: sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 (")" :: acc))
+      (* optimization to make use of n-ary disjunction/conjunction *)
+      (* prop_formula -> string list -> string list *)
+      and sat_string_acc_or (Or (fm1,fm2)) acc =
+        sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 acc)
+        | sat_string_acc_or fm acc =
+        sat_string_acc fm acc
+      (* prop_formula -> string list -> string list *)
+      and sat_string_acc_and (And (fm1,fm2)) acc =
+        sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 acc)
+        | sat_string_acc_and fm acc =
+        sat_string_acc fm acc
+    in
+      sat_string_acc fm []
+    end
+    val number_of_vars = Int.max (maxidx fm, 1)
+  in
+    File.write path
+      ("c This file was generated by SatSolver.write_dimacs_sat_file\n" ^
+       "p sat " ^ string_of_int number_of_vars ^ "\n" ^
+       "(");
+    File.append_list path
+      (sat_string fm);
+    File.append path
+      ")\n"
+  end;
 
 (* ------------------------------------------------------------------------- *)
 (* read_std_result_file: scans a SAT solver's output file for a satisfying   *)
 (*      variable assignment.  Returns the assignment, or 'UNSATISFIABLE' if  *)
 (*      the file contains 'unsatisfiable', or 'UNKNOWN' if the file contains *)

 (*      assignment, then i is interpreted as 'true'.  If ~i is contained in  *)
 (*      the assignment, then i is interpreted as 'false'.  Otherwise the     *)
 (*      value of i is taken to be unspecified.                               *)
 (* ------------------------------------------------------------------------- *)
 
-	(* Path.T -> string * string * string -> result *)
-
-	fun read_std_result_file path (satisfiable, assignment_prefix, unsatisfiable) =
-	let
-		(* string -> int list *)
-		fun int_list_from_string s =
-			List.mapPartial Int.fromString (space_explode " " s)
-		(* int list -> assignment *)
-		fun assignment_from_list [] i =
-			NONE  (* the SAT solver didn't provide a value for this variable *)
-		  | assignment_from_list (x::xs) i =
-			if x=i then (SOME true)
-			else if x=(~i) then (SOME false)
-			else assignment_from_list xs i
-		(* int list -> string list -> assignment *)
-		fun parse_assignment xs [] =
-			assignment_from_list xs
-		  | parse_assignment xs (line::lines) =
-			if String.isPrefix assignment_prefix line then
-				parse_assignment (xs @ int_list_from_string line) lines
-			else
-				assignment_from_list xs
-		(* string -> string -> bool *)
-		fun is_substring needle haystack =
-		let
-			val length1 = String.size needle
-			val length2 = String.size haystack
-		in
-			if length2 < length1 then
-				false
-			else if needle = String.substring (haystack, 0, length1) then
-				true
-			else is_substring needle (String.substring (haystack, 1, length2-1))
-		end
-		(* string list -> result *)
-		fun parse_lines [] =
-			UNKNOWN
-		  | parse_lines (line::lines) =
-			if is_substring unsatisfiable line then
-				UNSATISFIABLE NONE
-			else if is_substring satisfiable line then
-				SATISFIABLE (parse_assignment [] lines)
-			else
-				parse_lines lines
-	in
-		(parse_lines o (List.filter (fn l => l <> "")) o split_lines o File.read) path
-	end;
+  (* Path.T -> string * string * string -> result *)
+
+  fun read_std_result_file path (satisfiable, assignment_prefix, unsatisfiable) =
+  let
+    (* string -> int list *)
+    fun int_list_from_string s =
+      List.mapPartial Int.fromString (space_explode " " s)
+    (* int list -> assignment *)
+    fun assignment_from_list [] i =
+      NONE  (* the SAT solver didn't provide a value for this variable *)
+      | assignment_from_list (x::xs) i =
+      if x=i then (SOME true)
+      else if x=(~i) then (SOME false)
+      else assignment_from_list xs i
+    (* int list -> string list -> assignment *)
+    fun parse_assignment xs [] =
+      assignment_from_list xs
+      | parse_assignment xs (line::lines) =
+      if String.isPrefix assignment_prefix line then
+        parse_assignment (xs @ int_list_from_string line) lines
+      else
+        assignment_from_list xs
+    (* string -> string -> bool *)
+    fun is_substring needle haystack =
+    let
+      val length1 = String.size needle
+      val length2 = String.size haystack
+    in
+      if length2 < length1 then
+        false
+      else if needle = String.substring (haystack, 0, length1) then
+        true
+      else is_substring needle (String.substring (haystack, 1, length2-1))
+    end
+    (* string list -> result *)
+    fun parse_lines [] =
+      UNKNOWN
+      | parse_lines (line::lines) =
+      if is_substring unsatisfiable line then
+        UNSATISFIABLE NONE
+      else if is_substring satisfiable line then
+        SATISFIABLE (parse_assignment [] lines)
+      else
+        parse_lines lines
+  in
+    (parse_lines o (List.filter (fn l => l <> "")) o split_lines o File.read) path
+  end;
 
 (* ------------------------------------------------------------------------- *)
 (* make_external_solver: call 'writefn', execute 'cmd', call 'readfn'        *)
 (* ------------------------------------------------------------------------- *)
 
-	(* string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver *)
-
-	fun make_external_solver cmd writefn readfn fm =
-		(writefn fm; system cmd; readfn ());
+  (* string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver *)
+
+  fun make_external_solver cmd writefn readfn fm =
+    (writefn fm; system cmd; readfn ());
 
 (* ------------------------------------------------------------------------- *)
 (* read_dimacs_cnf_file: returns a propositional formula that corresponds to *)
 (*      a SAT problem given in DIMACS CNF format                             *)
 (* ------------------------------------------------------------------------- *)
 
-	(* Path.T -> PropLogic.prop_formula *)
-
-	fun read_dimacs_cnf_file path =
-	let
-		(* string list -> string list *)
-		fun filter_preamble [] =
-			error "problem line not found in DIMACS CNF file"
-		  | filter_preamble (line::lines) =
-			if String.isPrefix "c " line orelse line = "c" then
-				(* ignore comments *)
-				filter_preamble lines
-			else if String.isPrefix "p " line then
-				(* ignore the problem line (which must be the last line of the preamble) *)
-				(* Ignoring the problem line implies that if the file contains more clauses *)
-				(* or variables than specified in its preamble, we will accept it anyway.   *)
-				lines
-			else
-				error "preamble in DIMACS CNF file contains a line that does not begin with \"c \" or \"p \""
-		(* string -> int *)
-		fun int_from_string s =
-			case Int.fromString s of
-			  SOME i => i
-			| NONE   => error ("token " ^ quote s ^ "in DIMACS CNF file is not a number")
-		(* int list -> int list list *)
-		fun clauses xs =
-			let
-				val (xs1, xs2) = take_prefix (fn i => i <> 0) xs
-			in
-				case xs2 of
-				  []      => [xs1]
-				| (0::[]) => [xs1]
-				| (0::tl) => xs1 :: clauses tl
-				| _       => sys_error "this cannot happen"
-			end
-		(* int -> PropLogic.prop_formula *)
-		fun literal_from_int i =
-			(assert (i<>0) "variable index in DIMACS CNF file is 0";
-			if i>0 then
-				PropLogic.BoolVar i
-			else
-				PropLogic.Not (PropLogic.BoolVar (~i)))
-		(* PropLogic.prop_formula list -> PropLogic.prop_formula *)
-		fun disjunction [] =
-			error "empty clause in DIMACS CNF file"
-		  | disjunction (x::xs) =
-			(case xs of
-			  [] => x
-			| _  => PropLogic.Or (x, disjunction xs))
-		(* PropLogic.prop_formula list -> PropLogic.prop_formula *)
-		fun conjunction [] =
-			error "no clause in DIMACS CNF file"
-		  | conjunction (x::xs) =
-			(case xs of
-			  [] => x
-			| _  => PropLogic.And (x, conjunction xs))
-	in
-		(conjunction
-		o (map disjunction)
-		o (map (map literal_from_int))
-		o clauses
-		o (map int_from_string)
-		o List.concat
-		o (map (String.fields (fn c => c mem [#" ", #"\t", #"\n"])))
-		o filter_preamble
-		o (List.filter (fn l => l <> ""))
-		o split_lines
-		o File.read)
-			path
-	end;
+  (* Path.T -> PropLogic.prop_formula *)
+
+  fun read_dimacs_cnf_file path =
+  let
+    (* string list -> string list *)
+    fun filter_preamble [] =
+      error "problem line not found in DIMACS CNF file"
+      | filter_preamble (line::lines) =
+      if String.isPrefix "c " line orelse line = "c" then
+        (* ignore comments *)
+        filter_preamble lines
+      else if String.isPrefix "p " line then
+        (* ignore the problem line (which must be the last line of the preamble) *)
+        (* Ignoring the problem line implies that if the file contains more clauses *)
+        (* or variables than specified in its preamble, we will accept it anyway.   *)
+        lines
+      else
+        error "preamble in DIMACS CNF file contains a line that does not begin with \"c \" or \"p \""
+    (* string -> int *)
+    fun int_from_string s =
+      case Int.fromString s of
+        SOME i => i
+      | NONE   => error ("token " ^ quote s ^ "in DIMACS CNF file is not a number")
+    (* int list -> int list list *)
+    fun clauses xs =
+      let
+        val (xs1, xs2) = take_prefix (fn i => i <> 0) xs
+      in
+        case xs2 of
+          []      => [xs1]
+        | (0::[]) => [xs1]
+        | (0::tl) => xs1 :: clauses tl
+        | _       => sys_error "this cannot happen"
+      end
+    (* int -> PropLogic.prop_formula *)
+    fun literal_from_int i =
+      (i<>0 orelse error "variable index in DIMACS CNF file is 0";
+      if i>0 then
+        PropLogic.BoolVar i
+      else
+        PropLogic.Not (PropLogic.BoolVar (~i)))
+    (* PropLogic.prop_formula list -> PropLogic.prop_formula *)
+    fun disjunction [] =
+      error "empty clause in DIMACS CNF file"
+      | disjunction (x::xs) =
+      (case xs of
+        [] => x
+      | _  => PropLogic.Or (x, disjunction xs))
+    (* PropLogic.prop_formula list -> PropLogic.prop_formula *)
+    fun conjunction [] =
+      error "no clause in DIMACS CNF file"
+      | conjunction (x::xs) =
+      (case xs of
+        [] => x
+      | _  => PropLogic.And (x, conjunction xs))
+  in
+    (conjunction
+    o (map disjunction)
+    o (map (map literal_from_int))
+    o clauses
+    o (map int_from_string)
+    o List.concat
+    o (map (String.fields (fn c => c mem [#" ", #"\t", #"\n"])))
+    o filter_preamble
+    o (List.filter (fn l => l <> ""))
+    o split_lines
+    o File.read)
+      path
+  end;
 
 (* ------------------------------------------------------------------------- *)
 (* solvers: a (reference to a) table of all registered SAT solvers           *)
 (* ------------------------------------------------------------------------- *)
 
-	val solvers = ref ([] : (string * solver) list);
+  val solvers = ref ([] : (string * solver) list);
 
 (* ------------------------------------------------------------------------- *)
 (* add_solver: updates 'solvers' by adding a new solver                      *)
 (* ------------------------------------------------------------------------- *)
 
-	(* string * solver -> unit *)
+  (* string * solver -> unit *)
 
     fun add_solver (name, new_solver) =
       let
         val the_solvers = !solvers;
         val _ = if AList.defined (op =) the_solvers name

 (* invoke_solver: returns the solver associated with the given 'name'        *)
 (* Note: If no solver is associated with 'name', exception 'Option' will be  *)
 (*       raised.                                                             *)
 (* ------------------------------------------------------------------------- *)
 
-	(* string -> solver *)
-
-	fun invoke_solver name =
-		(the o AList.lookup (op =) (!solvers)) name;
+  (* string -> solver *)
+
+  fun invoke_solver name =
+    (the o AList.lookup (op =) (!solvers)) name;
 
 end;  (* SatSolver *)
 
 
 (* ------------------------------------------------------------------------- *)

 (* Internal SAT solver, available as 'SatSolver.invoke_solver "enumerate"'   *)
 (* -- simply enumerates assignments until a satisfying assignment is found   *)
 (* ------------------------------------------------------------------------- *)
 
 let
-	fun enum_solver fm =
-	let
-		(* int list *)
-		val indices = PropLogic.indices fm
-		(* int list -> int list -> int list option *)
-		(* binary increment: list 'xs' of current bits, list 'ys' of all bits (lower bits first) *)
-		fun next_list _ ([]:int list) =
-			NONE
-		  | next_list [] (y::ys) =
-			SOME [y]
-		  | next_list (x::xs) (y::ys) =
-			if x=y then
-				(* reset the bit, continue *)
-				next_list xs ys
-			else
-				(* set the lowest bit that wasn't set before, keep the higher bits *)
-				SOME (y::x::xs)
-		(* int list -> int -> bool *)
-		fun assignment_from_list xs i =
-			i mem xs
-		(* int list -> SatSolver.result *)
-		fun solver_loop xs =
-			if PropLogic.eval (assignment_from_list xs) fm then
-				SatSolver.SATISFIABLE (SOME o (assignment_from_list xs))
-			else
-				(case next_list xs indices of
-				  SOME xs' => solver_loop xs'
-				| NONE     => SatSolver.UNSATISFIABLE NONE)
-	in
-		(* start with the "first" assignment (all variables are interpreted as 'false') *)
-		solver_loop []
-	end
+  fun enum_solver fm =
+  let
+    (* int list *)
+    val indices = PropLogic.indices fm
+    (* int list -> int list -> int list option *)
+    (* binary increment: list 'xs' of current bits, list 'ys' of all bits (lower bits first) *)
+    fun next_list _ ([]:int list) =
+      NONE
+      | next_list [] (y::ys) =
+      SOME [y]
+      | next_list (x::xs) (y::ys) =
+      if x=y then
+        (* reset the bit, continue *)
+        next_list xs ys
+      else
+        (* set the lowest bit that wasn't set before, keep the higher bits *)
+        SOME (y::x::xs)
+    (* int list -> int -> bool *)
+    fun assignment_from_list xs i =
+      i mem xs
+    (* int list -> SatSolver.result *)
+    fun solver_loop xs =
+      if PropLogic.eval (assignment_from_list xs) fm then
+        SatSolver.SATISFIABLE (SOME o (assignment_from_list xs))
+      else
+        (case next_list xs indices of
+          SOME xs' => solver_loop xs'
+        | NONE     => SatSolver.UNSATISFIABLE NONE)
+  in
+    (* start with the "first" assignment (all variables are interpreted as 'false') *)
+    solver_loop []
+  end
 in
-	SatSolver.add_solver ("enumerate", enum_solver)
+  SatSolver.add_solver ("enumerate", enum_solver)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* Internal SAT solver, available as 'SatSolver.invoke_solver "dpll"' -- a   *)
 (* simple implementation of the DPLL algorithm (cf. L. Zhang, S. Malik: "The *)
 (* Quest for Efficient Boolean Satisfiability Solvers", July 2002, Fig. 1).  *)
 (* ------------------------------------------------------------------------- *)
 
 let
-	local
-		open PropLogic
-	in
-		fun dpll_solver fm =
-		let
-			(* We could use 'PropLogic.defcnf fm' instead of 'PropLogic.nnf fm' *)
-			(* but that sometimes leads to worse performance due to the         *)
-			(* introduction of additional variables.                            *)
-			val fm' = PropLogic.nnf fm
-			(* int list *)
-			val indices = PropLogic.indices fm'
-			(* int list -> int -> prop_formula *)
-			fun partial_var_eval []      i = BoolVar i
-			  | partial_var_eval (x::xs) i = if x=i then True else if x=(~i) then False else partial_var_eval xs i
-			(* int list -> prop_formula -> prop_formula *)
-			fun partial_eval xs True             = True
-			  | partial_eval xs False            = False
-			  | partial_eval xs (BoolVar i)      = partial_var_eval xs i
-			  | partial_eval xs (Not fm)         = SNot (partial_eval xs fm)
-			  | partial_eval xs (Or (fm1, fm2))  = SOr (partial_eval xs fm1, partial_eval xs fm2)
-			  | partial_eval xs (And (fm1, fm2)) = SAnd (partial_eval xs fm1, partial_eval xs fm2)
-			(* prop_formula -> int list *)
-			fun forced_vars True              = []
-			  | forced_vars False             = []
-			  | forced_vars (BoolVar i)       = [i]
-			  | forced_vars (Not (BoolVar i)) = [~i]
-			  | forced_vars (Or (fm1, fm2))   = (forced_vars fm1) inter_int (forced_vars fm2)
-			  | forced_vars (And (fm1, fm2))  = (forced_vars fm1) union_int (forced_vars fm2)
-			  (* Above, i *and* ~i may be forced.  In this case the first occurrence takes   *)
-			  (* precedence, and the next partial evaluation of the formula returns 'False'. *)
-			  | forced_vars _                 = error "formula is not in negation normal form"
-			(* int list -> prop_formula -> (int list * prop_formula) *)
-			fun eval_and_force xs fm =
-			let
-				val fm' = partial_eval xs fm
-				val xs' = forced_vars fm'
-			in
-				if null xs' then
-					(xs, fm')
-				else
-					eval_and_force (xs@xs') fm'  (* xs and xs' should be distinct, so '@' here should have *)
-					                             (* the same effect as 'union_int'                         *)
-			end
-			(* int list -> int option *)
-			fun fresh_var xs =
-				Library.find_first (fn i => not (i mem_int xs) andalso not ((~i) mem_int xs)) indices
-			(* int list -> prop_formula -> int list option *)
-			(* partial assignment 'xs' *)
-			fun dpll xs fm =
-			let
-				val (xs', fm') = eval_and_force xs fm
-			in
-				case fm' of
-				  True  => SOME xs'
-				| False => NONE
-				| _     =>
-					let
-						val x = valOf (fresh_var xs')  (* a fresh variable must exist since 'fm' did not evaluate to 'True'/'False' *)
-					in
-						case dpll (x::xs') fm' of  (* passing fm' rather than fm should save some simplification work *)
-						  SOME xs'' => SOME xs''
-						| NONE      => dpll ((~x)::xs') fm'  (* now try interpreting 'x' as 'False' *)
-					end
-			end
-			(* int list -> assignment *)
-			fun assignment_from_list [] i =
-				NONE  (* the DPLL procedure didn't provide a value for this variable *)
-			  | assignment_from_list (x::xs) i =
-				if x=i then (SOME true)
-				else if x=(~i) then (SOME false)
-				else assignment_from_list xs i
-		in
-			(* initially, no variable is interpreted yet *)
-			case dpll [] fm' of
-			  SOME assignment => SatSolver.SATISFIABLE (assignment_from_list assignment)
-			| NONE            => SatSolver.UNSATISFIABLE NONE
-		end
-	end  (* local *)
+  local
+    open PropLogic
+  in
+    fun dpll_solver fm =
+    let
+      (* We could use 'PropLogic.defcnf fm' instead of 'PropLogic.nnf fm' *)
+      (* but that sometimes leads to worse performance due to the         *)
+      (* introduction of additional variables.                            *)
+      val fm' = PropLogic.nnf fm
+      (* int list *)
+      val indices = PropLogic.indices fm'
+      (* int list -> int -> prop_formula *)
+      fun partial_var_eval []      i = BoolVar i
+        | partial_var_eval (x::xs) i = if x=i then True else if x=(~i) then False else partial_var_eval xs i
+      (* int list -> prop_formula -> prop_formula *)
+      fun partial_eval xs True             = True
+        | partial_eval xs False            = False
+        | partial_eval xs (BoolVar i)      = partial_var_eval xs i
+        | partial_eval xs (Not fm)         = SNot (partial_eval xs fm)
+        | partial_eval xs (Or (fm1, fm2))  = SOr (partial_eval xs fm1, partial_eval xs fm2)
+        | partial_eval xs (And (fm1, fm2)) = SAnd (partial_eval xs fm1, partial_eval xs fm2)
+      (* prop_formula -> int list *)
+      fun forced_vars True              = []
+        | forced_vars False             = []
+        | forced_vars (BoolVar i)       = [i]
+        | forced_vars (Not (BoolVar i)) = [~i]
+        | forced_vars (Or (fm1, fm2))   = (forced_vars fm1) inter_int (forced_vars fm2)
+        | forced_vars (And (fm1, fm2))  = (forced_vars fm1) union_int (forced_vars fm2)
+        (* Above, i *and* ~i may be forced.  In this case the first occurrence takes   *)
+        (* precedence, and the next partial evaluation of the formula returns 'False'. *)
+        | forced_vars _                 = error "formula is not in negation normal form"
+      (* int list -> prop_formula -> (int list * prop_formula) *)
+      fun eval_and_force xs fm =
+      let
+        val fm' = partial_eval xs fm
+        val xs' = forced_vars fm'
+      in
+        if null xs' then
+          (xs, fm')
+        else
+          eval_and_force (xs@xs') fm'  (* xs and xs' should be distinct, so '@' here should have *)
+                                       (* the same effect as 'union_int'                         *)
+      end
+      (* int list -> int option *)
+      fun fresh_var xs =
+        Library.find_first (fn i => not (i mem_int xs) andalso not ((~i) mem_int xs)) indices
+      (* int list -> prop_formula -> int list option *)
+      (* partial assignment 'xs' *)
+      fun dpll xs fm =
+      let
+        val (xs', fm') = eval_and_force xs fm
+      in
+        case fm' of
+          True  => SOME xs'
+        | False => NONE
+        | _     =>
+          let
+            val x = valOf (fresh_var xs')  (* a fresh variable must exist since 'fm' did not evaluate to 'True'/'False' *)
+          in
+            case dpll (x::xs') fm' of  (* passing fm' rather than fm should save some simplification work *)
+              SOME xs'' => SOME xs''
+            | NONE      => dpll ((~x)::xs') fm'  (* now try interpreting 'x' as 'False' *)
+          end
+      end
+      (* int list -> assignment *)
+      fun assignment_from_list [] i =
+        NONE  (* the DPLL procedure didn't provide a value for this variable *)
+        | assignment_from_list (x::xs) i =
+        if x=i then (SOME true)
+        else if x=(~i) then (SOME false)
+        else assignment_from_list xs i
+    in
+      (* initially, no variable is interpreted yet *)
+      case dpll [] fm' of
+        SOME assignment => SatSolver.SATISFIABLE (assignment_from_list assignment)
+      | NONE            => SatSolver.UNSATISFIABLE NONE
+    end
+  end  (* local *)
 in
-	SatSolver.add_solver ("dpll", dpll_solver)
+  SatSolver.add_solver ("dpll", dpll_solver)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* Internal SAT solver, available as 'SatSolver.invoke_solver "auto"': uses  *)
 (* the last installed solver (other than "auto" itself) that does not raise  *)
 (* 'NOT_CONFIGURED'.  (However, the solver may return 'UNKNOWN'.)            *)
 (* ------------------------------------------------------------------------- *)
 
 let
-	fun auto_solver fm =
-	let
-		fun loop [] =
-			SatSolver.UNKNOWN
-		  | loop ((name, solver)::solvers) =
-			if name="auto" then
-				(* do not call solver "auto" from within "auto" *)
-				loop solvers
-			else (
-				(if name="dpll" orelse name="enumerate" then
-					warning ("Using SAT solver " ^ quote name ^ "; for better performance, consider using an external solver.")
-				else
-					tracing ("Using SAT solver " ^ quote name ^ "."));
-				(* apply 'solver' to 'fm' *)
-				solver fm
-					handle SatSolver.NOT_CONFIGURED => loop solvers
-			)
-	in
-		loop (!SatSolver.solvers)
-	end
+  fun auto_solver fm =
+  let
+    fun loop [] =
+      SatSolver.UNKNOWN
+      | loop ((name, solver)::solvers) =
+      if name="auto" then
+        (* do not call solver "auto" from within "auto" *)
+        loop solvers
+      else (
+        (if name="dpll" orelse name="enumerate" then
+          warning ("Using SAT solver " ^ quote name ^ "; for better performance, consider using an external solver.")
+        else
+          tracing ("Using SAT solver " ^ quote name ^ "."));
+        (* apply 'solver' to 'fm' *)
+        solver fm
+          handle SatSolver.NOT_CONFIGURED => loop solvers
+      )
+  in
+    loop (!SatSolver.solvers)
+  end
 in
-	SatSolver.add_solver ("auto", auto_solver)
+  SatSolver.add_solver ("auto", auto_solver)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* MiniSat 1.14                                                              *)
 (* (http://www.cs.chalmers.se/Cs/Research/FormalMethods/MiniSat/)            *)

 (* introduces IDs for original clauses in the proof trace.  It does not (in  *)
 (* general) follow the convention that the original clauses are numbered     *)
 (* from 0 to n-1 (where n is the number of clauses in the formula).          *)
 
 let
-	exception INVALID_PROOF of string
-	fun minisat_with_proofs fm =
-	let
-		val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
-		val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
-		val outpath    = File.tmp_path (Path.explode "result")
-		val proofpath  = File.tmp_path (Path.explode "result.prf")
-		val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " -t " ^ (Path.implode proofpath) ^ "> /dev/null"
-		fun writefn fm = SatSolver.write_dimacs_cnf_file inpath fm
-		fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")
-		val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
-		val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
-		val cnf        = PropLogic.defcnf fm
-		val result     = SatSolver.make_external_solver cmd writefn readfn cnf
-		val _          = try File.rm inpath
-		val _          = try File.rm outpath
-	in  case result of
-	  SatSolver.UNSATISFIABLE NONE =>
-		(let
-			(* string list *)
-			val proof_lines = (split_lines o File.read) proofpath
-				handle IO.Io _ => raise INVALID_PROOF "Could not read file \"result.prf\""
-			(* representation of clauses as ordered lists of literals (with duplicates removed) *)
-			(* prop_formula -> int list *)
-			fun clause_to_lit_list (PropLogic.Or (fm1, fm2)) =
-				OrdList.union int_ord (clause_to_lit_list fm1) (clause_to_lit_list fm2)
-			  | clause_to_lit_list (PropLogic.BoolVar i) =
-				[i]
-			  | clause_to_lit_list (PropLogic.Not (PropLogic.BoolVar i)) =
-				[~i]
-			  | clause_to_lit_list _ =
-				raise INVALID_PROOF "Error: invalid clause in CNF formula."
-			(* prop_formula -> int *)
-			fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) =
-				cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2
-			  | cnf_number_of_clauses _ =
-				1
-			val number_of_clauses = cnf_number_of_clauses cnf
-			(* int list array *)
-			val clauses = Array.array (number_of_clauses, [])
-			(* initialize the 'clauses' array *)
-			(* prop_formula * int -> int *)
-			fun init_array (PropLogic.And (fm1, fm2), n) =
-				init_array (fm2, init_array (fm1, n))
-			  | init_array (fm, n) =
-				(Array.update (clauses, n, clause_to_lit_list fm); n+1)
-			val _ = init_array (cnf, 0)
-			(* optimization for the common case where MiniSat "R"s clauses in their *)
-			(* original order:                                                      *)
-			val last_ref_clause = ref (number_of_clauses - 1)
-			(* search the 'clauses' array for the given list of literals 'lits', *)
-			(* starting at index '!last_ref_clause + 1'                          *)
-			(* int list -> int option *)
-			fun original_clause_id lits =
-			let
-				fun original_clause_id_from index =
-					if index = number_of_clauses then
-						(* search from beginning again *)
-						original_clause_id_from 0
-					(* both 'lits' and the list of literals used in 'clauses' are sorted, so *)
-					(* testing for equality should suffice -- barring duplicate literals     *)
-					else if Array.sub (clauses, index) = lits then (
-						(* success *)
-						last_ref_clause := index;
-						SOME index
-					) else if index = !last_ref_clause then
-						(* failure *)
-						NONE
-					else
-						(* continue search *)
-						original_clause_id_from (index + 1)
-			in
-				original_clause_id_from (!last_ref_clause + 1)
-			end
-			(* string -> int *)
-			fun int_from_string s = (
-				case Int.fromString s of
-				  SOME i => i
-				| NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")
-			)
-			(* parse the proof file *)
-			val clause_table  = ref (Inttab.empty : int list Inttab.table)
-			val empty_id      = ref ~1
-			(* contains a mapping from clause IDs as used by MiniSat to clause IDs in *)
-			(* our proof format, where original clauses are numbered starting from 0  *)
-			val clause_id_map = ref (Inttab.empty : int Inttab.table)
-			fun sat_to_proof id = (
-				case Inttab.lookup (!clause_id_map) id of
-				  SOME id' => id'
-				| NONE     => raise INVALID_PROOF ("Clause ID " ^ Int.toString id ^ " used, but not defined.")
-			)
-			val next_id = ref (number_of_clauses - 1)
-			(* string list -> unit *)
-			fun process_tokens [] =
-				()
-			  | process_tokens (tok::toks) =
-				if tok="R" then (
-					case toks of
-					  id::sep::lits =>
-						let
-							val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"R\" disallowed after \"X\"."
-							val cid      = int_from_string id
-							val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
-							val ls       = sort int_ord (map int_from_string lits)
-							val proof_id = case original_clause_id ls of
-							                 SOME orig_id => orig_id
-							               | NONE         => raise INVALID_PROOF ("Original clause (new ID is " ^ id ^ ") not found.")
-						in
-							(* extend the mapping of clause IDs with this newly defined ID *)
-							clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)
-								handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"R\").")
-							(* the proof itself doesn't change *)
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"R\" followed by an insufficient number of tokens."
-				) else if tok="C" then (
-					case toks of
-					  id::sep::ids =>
-						let
-							val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"C\" disallowed after \"X\"."
-							val cid      = int_from_string id
-							val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
-							(* ignore the pivot literals in MiniSat's trace *)
-							fun unevens []             = raise INVALID_PROOF "File format error: \"C\" followed by an even number of IDs."
-							  | unevens (x :: [])      = x :: []
-							  | unevens (x :: _ :: xs) = x :: unevens xs
-							val rs       = (map sat_to_proof o unevens o map int_from_string) ids
-							(* extend the mapping of clause IDs with this newly defined ID *)
-							val proof_id = inc next_id
-							val _        = clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)
-							                 handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"C\").")
-						in
-							(* update clause table *)
-							clause_table := Inttab.update_new (proof_id, rs) (!clause_table)
-								handle Inttab.DUP _ => raise INVALID_PROOF ("Error: internal ID for clause " ^ id ^ " already used.")
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"C\" followed by an insufficient number of tokens."
-				) else if tok="D" then (
-					case toks of
-					  [id] =>
-						let
-							val _ = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"D\" disallowed after \"X\"."
-							val _ = sat_to_proof (int_from_string id)
-						in
-							(* simply ignore "D" *)
-							()
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"D\" followed by an illegal number of tokens."
-				) else if tok="X" then (
-					case toks of
-					  [id1, id2] =>
-						let
-							val _            = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one end-of-proof statement."
-							val _            = sat_to_proof (int_from_string id1)
-							val new_empty_id = sat_to_proof (int_from_string id2)
-						in
-							(* update conflict id *)
-							empty_id := new_empty_id
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"X\" followed by an illegal number of tokens."
-				) else
-					raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")
-			(* string list -> unit *)
-			fun process_lines [] =
-				()
-			  | process_lines (l::ls) = (
-					process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);
-					process_lines ls
-				)
-			(* proof *)
-			val _ = process_lines proof_lines
-			val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."
-		in
-			SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))
-		end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))
-	| result =>
-		result
-	end
+  exception INVALID_PROOF of string
+  fun minisat_with_proofs fm =
+  let
+    val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
+    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
+    val outpath    = File.tmp_path (Path.explode "result")
+    val proofpath  = File.tmp_path (Path.explode "result.prf")
+    val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " -t " ^ (Path.implode proofpath) ^ "> /dev/null"
+    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath fm
+    fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")
+    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
+    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
+    val cnf        = PropLogic.defcnf fm
+    val result     = SatSolver.make_external_solver cmd writefn readfn cnf
+    val _          = try File.rm inpath
+    val _          = try File.rm outpath
+  in  case result of
+    SatSolver.UNSATISFIABLE NONE =>
+    (let
+      (* string list *)
+      val proof_lines = (split_lines o File.read) proofpath
+        handle IO.Io _ => raise INVALID_PROOF "Could not read file \"result.prf\""
+      (* representation of clauses as ordered lists of literals (with duplicates removed) *)
+      (* prop_formula -> int list *)
+      fun clause_to_lit_list (PropLogic.Or (fm1, fm2)) =
+        OrdList.union int_ord (clause_to_lit_list fm1) (clause_to_lit_list fm2)
+        | clause_to_lit_list (PropLogic.BoolVar i) =
+        [i]
+        | clause_to_lit_list (PropLogic.Not (PropLogic.BoolVar i)) =
+        [~i]
+        | clause_to_lit_list _ =
+        raise INVALID_PROOF "Error: invalid clause in CNF formula."
+      (* prop_formula -> int *)
+      fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) =
+        cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2
+        | cnf_number_of_clauses _ =
+        1
+      val number_of_clauses = cnf_number_of_clauses cnf
+      (* int list array *)
+      val clauses = Array.array (number_of_clauses, [])
+      (* initialize the 'clauses' array *)
+      (* prop_formula * int -> int *)
+      fun init_array (PropLogic.And (fm1, fm2), n) =
+        init_array (fm2, init_array (fm1, n))
+        | init_array (fm, n) =
+        (Array.update (clauses, n, clause_to_lit_list fm); n+1)
+      val _ = init_array (cnf, 0)
+      (* optimization for the common case where MiniSat "R"s clauses in their *)
+      (* original order:                                                      *)
+      val last_ref_clause = ref (number_of_clauses - 1)
+      (* search the 'clauses' array for the given list of literals 'lits', *)
+      (* starting at index '!last_ref_clause + 1'                          *)
+      (* int list -> int option *)
+      fun original_clause_id lits =
+      let
+        fun original_clause_id_from index =
+          if index = number_of_clauses then
+            (* search from beginning again *)
+            original_clause_id_from 0
+          (* both 'lits' and the list of literals used in 'clauses' are sorted, so *)
+          (* testing for equality should suffice -- barring duplicate literals     *)
+          else if Array.sub (clauses, index) = lits then (
+            (* success *)
+            last_ref_clause := index;
+            SOME index
+          ) else if index = !last_ref_clause then
+            (* failure *)
+            NONE
+          else
+            (* continue search *)
+            original_clause_id_from (index + 1)
+      in
+        original_clause_id_from (!last_ref_clause + 1)
+      end
+      (* string -> int *)
+      fun int_from_string s = (
+        case Int.fromString s of
+          SOME i => i
+        | NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")
+      )
+      (* parse the proof file *)
+      val clause_table  = ref (Inttab.empty : int list Inttab.table)
+      val empty_id      = ref ~1
+      (* contains a mapping from clause IDs as used by MiniSat to clause IDs in *)
+      (* our proof format, where original clauses are numbered starting from 0  *)
+      val clause_id_map = ref (Inttab.empty : int Inttab.table)
+      fun sat_to_proof id = (
+        case Inttab.lookup (!clause_id_map) id of
+          SOME id' => id'
+        | NONE     => raise INVALID_PROOF ("Clause ID " ^ Int.toString id ^ " used, but not defined.")
+      )
+      val next_id = ref (number_of_clauses - 1)
+      (* string list -> unit *)
+      fun process_tokens [] =
+        ()
+        | process_tokens (tok::toks) =
+        if tok="R" then (
+          case toks of
+            id::sep::lits =>
+            let
+              val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"R\" disallowed after \"X\"."
+              val cid      = int_from_string id
+              val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
+              val ls       = sort int_ord (map int_from_string lits)
+              val proof_id = case original_clause_id ls of
+                               SOME orig_id => orig_id
+                             | NONE         => raise INVALID_PROOF ("Original clause (new ID is " ^ id ^ ") not found.")
+            in
+              (* extend the mapping of clause IDs with this newly defined ID *)
+              clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)
+                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"R\").")
+              (* the proof itself doesn't change *)
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"R\" followed by an insufficient number of tokens."
+        ) else if tok="C" then (
+          case toks of
+            id::sep::ids =>
+            let
+              val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"C\" disallowed after \"X\"."
+              val cid      = int_from_string id
+              val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
+              (* ignore the pivot literals in MiniSat's trace *)
+              fun unevens []             = raise INVALID_PROOF "File format error: \"C\" followed by an even number of IDs."
+                | unevens (x :: [])      = x :: []
+                | unevens (x :: _ :: xs) = x :: unevens xs
+              val rs       = (map sat_to_proof o unevens o map int_from_string) ids
+              (* extend the mapping of clause IDs with this newly defined ID *)
+              val proof_id = inc next_id
+              val _        = clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)
+                               handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"C\").")
+            in
+              (* update clause table *)
+              clause_table := Inttab.update_new (proof_id, rs) (!clause_table)
+                handle Inttab.DUP _ => raise INVALID_PROOF ("Error: internal ID for clause " ^ id ^ " already used.")
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"C\" followed by an insufficient number of tokens."
+        ) else if tok="D" then (
+          case toks of
+            [id] =>
+            let
+              val _ = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"D\" disallowed after \"X\"."
+              val _ = sat_to_proof (int_from_string id)
+            in
+              (* simply ignore "D" *)
+              ()
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"D\" followed by an illegal number of tokens."
+        ) else if tok="X" then (
+          case toks of
+            [id1, id2] =>
+            let
+              val _            = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one end-of-proof statement."
+              val _            = sat_to_proof (int_from_string id1)
+              val new_empty_id = sat_to_proof (int_from_string id2)
+            in
+              (* update conflict id *)
+              empty_id := new_empty_id
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"X\" followed by an illegal number of tokens."
+        ) else
+          raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")
+      (* string list -> unit *)
+      fun process_lines [] =
+        ()
+        | process_lines (l::ls) = (
+          process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);
+          process_lines ls
+        )
+      (* proof *)
+      val _ = process_lines proof_lines
+      val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."
+    in
+      SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))
+    end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))
+  | result =>
+    result
+  end
 in
-	SatSolver.add_solver ("minisat_with_proofs", minisat_with_proofs)
+  SatSolver.add_solver ("minisat_with_proofs", minisat_with_proofs)
 end;
 
 let
-	fun minisat fm =
-	let
-		val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
-		val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
-		val outpath    = File.tmp_path (Path.explode "result")
-		val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " > /dev/null"
-		fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
-		fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")
-		val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
-		val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
-		val result     = SatSolver.make_external_solver cmd writefn readfn fm
-		val _          = try File.rm inpath
-		val _          = try File.rm outpath
-	in
-		result
-	end
+  fun minisat fm =
+  let
+    val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
+    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
+    val outpath    = File.tmp_path (Path.explode "result")
+    val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " > /dev/null"
+    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
+    fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")
+    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
+    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
+    val result     = SatSolver.make_external_solver cmd writefn readfn fm
+    val _          = try File.rm inpath
+    val _          = try File.rm outpath
+  in
+    result
+  end
 in
-	SatSolver.add_solver ("minisat", minisat)
+  SatSolver.add_solver ("minisat", minisat)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* zChaff (http://www.princeton.edu/~chaff/zchaff.html)                      *)
 (* ------------------------------------------------------------------------- *)

 
 (* add "zchaff_with_proofs" _before_ "zchaff" to the available solvers, so   *)
 (* that the latter is preferred by the "auto" solver                         *)
 
 let
-	exception INVALID_PROOF of string
-	fun zchaff_with_proofs fm =
-	case SatSolver.invoke_solver "zchaff" fm of
-	  SatSolver.UNSATISFIABLE NONE =>
-		(let
-			(* string list *)
-			val proof_lines = ((split_lines o File.read) (Path.explode "resolve_trace"))
-				handle IO.Io _ => raise INVALID_PROOF "Could not read file \"resolve_trace\""
-			(* PropLogic.prop_formula -> int *)
-			fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) = cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2
-			  | cnf_number_of_clauses _                          = 1
-			(* string -> int *)
-			fun int_from_string s = (
-				case Int.fromString s of
-				  SOME i => i
-				| NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")
-			)
-			(* parse the "resolve_trace" file *)
-			val clause_offset = ref ~1
-			val clause_table  = ref (Inttab.empty : int list Inttab.table)
-			val empty_id      = ref ~1
-			(* string list -> unit *)
-			fun process_tokens [] =
-				()
-			  | process_tokens (tok::toks) =
-				if tok="CL:" then (
-					case toks of
-					  id::sep::ids =>
-						let
-							val _   = if !clause_offset = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"VAR:\".")
-							val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"CONF:\".")
-							val cid = int_from_string id
-							val _   = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
-							val rs  = map int_from_string ids
-						in
-							(* update clause table *)
-							clause_table := Inttab.update_new (cid, rs) (!clause_table)
-								handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once.")
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"CL:\" followed by an insufficient number of tokens."
-				) else if tok="VAR:" then (
-					case toks of
-					  id::levsep::levid::valsep::valid::antesep::anteid::litsep::lits =>
-						let
-							val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"VAR:\" disallowed after \"CONF:\".")
-							(* set 'clause_offset' to the largest used clause ID *)
-							val _   = if !clause_offset = ~1 then clause_offset :=
-								(case Inttab.max_key (!clause_table) of
-								  SOME id => id
-								| NONE    => cnf_number_of_clauses (PropLogic.defcnf fm) - 1  (* the first clause ID is 0, not 1 *))
-								else
-									()
-							val vid = int_from_string id
-							val _   = if levsep = "L:" then () else raise INVALID_PROOF ("File format error: \"L:\" expected (" ^ quote levsep ^ " encountered).")
-							val _   = int_from_string levid
-							val _   = if valsep = "V:" then () else raise INVALID_PROOF ("File format error: \"V:\" expected (" ^ quote valsep ^ " encountered).")
-							val _   = int_from_string valid
-							val _   = if antesep = "A:" then () else raise INVALID_PROOF ("File format error: \"A:\" expected (" ^ quote antesep ^ " encountered).")
-							val aid = int_from_string anteid
-							val _   = if litsep = "Lits:" then () else raise INVALID_PROOF ("File format error: \"Lits:\" expected (" ^ quote litsep ^ " encountered).")
-							val ls  = map int_from_string lits
-							(* convert the data provided by zChaff to our resolution-style proof format *)
-							(* each "VAR:" line defines a unit clause, the resolvents are implicitly    *)
-							(* given by the literals in the antecedent clause                           *)
-							(* we use the sum of '!clause_offset' and the variable ID as clause ID for the unit clause *)
-							val cid = !clause_offset + vid
-							(* the low bit of each literal gives its sign (positive/negative), therefore  *)
-							(* we have to divide each literal by 2 to obtain the proper variable ID; then *)
-							(* we add '!clause_offset' to obtain the ID of the corresponding unit clause  *)
-							val vids = filter (not_equal vid) (map (fn l => l div 2) ls)
-							val rs   = aid :: map (fn v => !clause_offset + v) vids
-						in
-							(* update clause table *)
-							clause_table := Inttab.update_new (cid, rs) (!clause_table)
-								handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int cid ^ " (derived from antecedent for variable " ^ id ^ ") already defined.")
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"VAR:\" followed by an insufficient number of tokens."
-				) else if tok="CONF:" then (
-					case toks of
-					  id::sep::ids =>
-						let
-							val _   = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one conflicting clause specified."
-							val cid = int_from_string id
-							val _   = if sep = "==" then () else raise INVALID_PROOF ("File format error: \"==\" expected (" ^ quote sep ^ " encountered).")
-							val ls  = map int_from_string ids
-							(* the conflict clause must be resolved with the unit clauses *)
-							(* for its literals to obtain the empty clause                *)
-							val vids         = map (fn l => l div 2) ls
-							val rs           = cid :: map (fn v => !clause_offset + v) vids
-							val new_empty_id = getOpt (Inttab.max_key (!clause_table), !clause_offset) + 1
-						in
-							(* update clause table and conflict id *)
-							clause_table := Inttab.update_new (new_empty_id, rs) (!clause_table)
-								handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int new_empty_id ^ " (empty clause derived from clause " ^ id ^ ") already defined.");
-							empty_id     := new_empty_id
-						end
-					| _ =>
-						raise INVALID_PROOF "File format error: \"CONF:\" followed by an insufficient number of tokens."
-				) else
-					raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")
-			(* string list -> unit *)
-			fun process_lines [] =
-				()
-			  | process_lines (l::ls) = (
-					process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);
-					process_lines ls
-				)
-			(* proof *)
-			val _ = process_lines proof_lines
-			val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."
-		in
-			SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))
-		end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))
-	| result =>
-		result
+  exception INVALID_PROOF of string
+  fun zchaff_with_proofs fm =
+  case SatSolver.invoke_solver "zchaff" fm of
+    SatSolver.UNSATISFIABLE NONE =>
+    (let
+      (* string list *)
+      val proof_lines = ((split_lines o File.read) (Path.explode "resolve_trace"))
+        handle IO.Io _ => raise INVALID_PROOF "Could not read file \"resolve_trace\""
+      (* PropLogic.prop_formula -> int *)
+      fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) = cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2
+        | cnf_number_of_clauses _                          = 1
+      (* string -> int *)
+      fun int_from_string s = (
+        case Int.fromString s of
+          SOME i => i
+        | NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")
+      )
+      (* parse the "resolve_trace" file *)
+      val clause_offset = ref ~1
+      val clause_table  = ref (Inttab.empty : int list Inttab.table)
+      val empty_id      = ref ~1
+      (* string list -> unit *)
+      fun process_tokens [] =
+        ()
+        | process_tokens (tok::toks) =
+        if tok="CL:" then (
+          case toks of
+            id::sep::ids =>
+            let
+              val _   = if !clause_offset = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"VAR:\".")
+              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"CONF:\".")
+              val cid = int_from_string id
+              val _   = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")
+              val rs  = map int_from_string ids
+            in
+              (* update clause table *)
+              clause_table := Inttab.update_new (cid, rs) (!clause_table)
+                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once.")
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"CL:\" followed by an insufficient number of tokens."
+        ) else if tok="VAR:" then (
+          case toks of
+            id::levsep::levid::valsep::valid::antesep::anteid::litsep::lits =>
+            let
+              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"VAR:\" disallowed after \"CONF:\".")
+              (* set 'clause_offset' to the largest used clause ID *)
+              val _   = if !clause_offset = ~1 then clause_offset :=
+                (case Inttab.max_key (!clause_table) of
+                  SOME id => id
+                | NONE    => cnf_number_of_clauses (PropLogic.defcnf fm) - 1  (* the first clause ID is 0, not 1 *))
+                else
+                  ()
+              val vid = int_from_string id
+              val _   = if levsep = "L:" then () else raise INVALID_PROOF ("File format error: \"L:\" expected (" ^ quote levsep ^ " encountered).")
+              val _   = int_from_string levid
+              val _   = if valsep = "V:" then () else raise INVALID_PROOF ("File format error: \"V:\" expected (" ^ quote valsep ^ " encountered).")
+              val _   = int_from_string valid
+              val _   = if antesep = "A:" then () else raise INVALID_PROOF ("File format error: \"A:\" expected (" ^ quote antesep ^ " encountered).")
+              val aid = int_from_string anteid
+              val _   = if litsep = "Lits:" then () else raise INVALID_PROOF ("File format error: \"Lits:\" expected (" ^ quote litsep ^ " encountered).")
+              val ls  = map int_from_string lits
+              (* convert the data provided by zChaff to our resolution-style proof format *)
+              (* each "VAR:" line defines a unit clause, the resolvents are implicitly    *)
+              (* given by the literals in the antecedent clause                           *)
+              (* we use the sum of '!clause_offset' and the variable ID as clause ID for the unit clause *)
+              val cid = !clause_offset + vid
+              (* the low bit of each literal gives its sign (positive/negative), therefore  *)
+              (* we have to divide each literal by 2 to obtain the proper variable ID; then *)
+              (* we add '!clause_offset' to obtain the ID of the corresponding unit clause  *)
+              val vids = filter (not_equal vid) (map (fn l => l div 2) ls)
+              val rs   = aid :: map (fn v => !clause_offset + v) vids
+            in
+              (* update clause table *)
+              clause_table := Inttab.update_new (cid, rs) (!clause_table)
+                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int cid ^ " (derived from antecedent for variable " ^ id ^ ") already defined.")
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"VAR:\" followed by an insufficient number of tokens."
+        ) else if tok="CONF:" then (
+          case toks of
+            id::sep::ids =>
+            let
+              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one conflicting clause specified."
+              val cid = int_from_string id
+              val _   = if sep = "==" then () else raise INVALID_PROOF ("File format error: \"==\" expected (" ^ quote sep ^ " encountered).")
+              val ls  = map int_from_string ids
+              (* the conflict clause must be resolved with the unit clauses *)
+              (* for its literals to obtain the empty clause                *)
+              val vids         = map (fn l => l div 2) ls
+              val rs           = cid :: map (fn v => !clause_offset + v) vids
+              val new_empty_id = getOpt (Inttab.max_key (!clause_table), !clause_offset) + 1
+            in
+              (* update clause table and conflict id *)
+              clause_table := Inttab.update_new (new_empty_id, rs) (!clause_table)
+                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int new_empty_id ^ " (empty clause derived from clause " ^ id ^ ") already defined.");
+              empty_id     := new_empty_id
+            end
+          | _ =>
+            raise INVALID_PROOF "File format error: \"CONF:\" followed by an insufficient number of tokens."
+        ) else
+          raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")
+      (* string list -> unit *)
+      fun process_lines [] =
+        ()
+        | process_lines (l::ls) = (
+          process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);
+          process_lines ls
+        )
+      (* proof *)
+      val _ = process_lines proof_lines
+      val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."
+    in
+      SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))
+    end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))
+  | result =>
+    result
 in
-	SatSolver.add_solver ("zchaff_with_proofs", zchaff_with_proofs)
+  SatSolver.add_solver ("zchaff_with_proofs", zchaff_with_proofs)
 end;
 
 let
-	fun zchaff fm =
-	let
-		val _          = if (getenv "ZCHAFF_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
-		val _          = if (getenv "ZCHAFF_VERSION") <> "2004.5.13" andalso
-		                    (getenv "ZCHAFF_VERSION") <> "2004.11.15" then raise SatSolver.NOT_CONFIGURED else ()
-			(* both versions of zChaff appear to have the same interface, so we do *)
-			(* not actually need to distinguish between them in the following code *)
-		val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
-		val outpath    = File.tmp_path (Path.explode "result")
-		val cmd        = (getenv "ZCHAFF_HOME") ^ "/zchaff " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
-		fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
-		fun readfn ()  = SatSolver.read_std_result_file outpath ("Instance Satisfiable", "", "Instance Unsatisfiable")
-		val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
-		val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
-		val result     = SatSolver.make_external_solver cmd writefn readfn fm
-		val _          = try File.rm inpath
-		val _          = try File.rm outpath
-	in
-		result
-	end
+  fun zchaff fm =
+  let
+    val _          = if (getenv "ZCHAFF_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
+    val _          = if (getenv "ZCHAFF_VERSION") <> "2004.5.13" andalso
+                        (getenv "ZCHAFF_VERSION") <> "2004.11.15" then raise SatSolver.NOT_CONFIGURED else ()
+      (* both versions of zChaff appear to have the same interface, so we do *)
+      (* not actually need to distinguish between them in the following code *)
+    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
+    val outpath    = File.tmp_path (Path.explode "result")
+    val cmd        = (getenv "ZCHAFF_HOME") ^ "/zchaff " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
+    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
+    fun readfn ()  = SatSolver.read_std_result_file outpath ("Instance Satisfiable", "", "Instance Unsatisfiable")
+    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
+    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
+    val result     = SatSolver.make_external_solver cmd writefn readfn fm
+    val _          = try File.rm inpath
+    val _          = try File.rm outpath
+  in
+    result
+  end
 in
-	SatSolver.add_solver ("zchaff", zchaff)
+  SatSolver.add_solver ("zchaff", zchaff)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* BerkMin 561 (http://eigold.tripod.com/BerkMin.html)                       *)
 (* ------------------------------------------------------------------------- *)
 
 let
-	fun berkmin fm =
-	let
-		val _          = if (getenv "BERKMIN_HOME") = "" orelse (getenv "BERKMIN_EXE") = "" then raise SatSolver.NOT_CONFIGURED else ()
-		val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
-		val outpath    = File.tmp_path (Path.explode "result")
-		val cmd        = (getenv "BERKMIN_HOME") ^ "/" ^ (getenv "BERKMIN_EXE") ^ " " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
-		fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
-		fun readfn ()  = SatSolver.read_std_result_file outpath ("Satisfiable          !!", "solution =", "UNSATISFIABLE          !!")
-		val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
-		val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
-		val result     = SatSolver.make_external_solver cmd writefn readfn fm
-		val _          = try File.rm inpath
-		val _          = try File.rm outpath
-	in
-		result
-	end
+  fun berkmin fm =
+  let
+    val _          = if (getenv "BERKMIN_HOME") = "" orelse (getenv "BERKMIN_EXE") = "" then raise SatSolver.NOT_CONFIGURED else ()
+    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
+    val outpath    = File.tmp_path (Path.explode "result")
+    val cmd        = (getenv "BERKMIN_HOME") ^ "/" ^ (getenv "BERKMIN_EXE") ^ " " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
+    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
+    fun readfn ()  = SatSolver.read_std_result_file outpath ("Satisfiable          !!", "solution =", "UNSATISFIABLE          !!")
+    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
+    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
+    val result     = SatSolver.make_external_solver cmd writefn readfn fm
+    val _          = try File.rm inpath
+    val _          = try File.rm outpath
+  in
+    result
+  end
 in
-	SatSolver.add_solver ("berkmin", berkmin)
+  SatSolver.add_solver ("berkmin", berkmin)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* Jerusat 1.3 (http://www.cs.tau.ac.il/~ale1/)                              *)
 (* ------------------------------------------------------------------------- *)
 
 let
-	fun jerusat fm =
-	let
-		val _          = if (getenv "JERUSAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
-		val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
-		val outpath    = File.tmp_path (Path.explode "result")
-		val cmd        = (getenv "JERUSAT_HOME") ^ "/Jerusat1.3 " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
-		fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
-		fun readfn ()  = SatSolver.read_std_result_file outpath ("s SATISFIABLE", "v ", "s UNSATISFIABLE")
-		val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
-		val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
-		val result     = SatSolver.make_external_solver cmd writefn readfn fm
-		val _          = try File.rm inpath
-		val _          = try File.rm outpath
-	in
-		result
-	end
+  fun jerusat fm =
+  let
+    val _          = if (getenv "JERUSAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()
+    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")
+    val outpath    = File.tmp_path (Path.explode "result")
+    val cmd        = (getenv "JERUSAT_HOME") ^ "/Jerusat1.3 " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)
+    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)
+    fun readfn ()  = SatSolver.read_std_result_file outpath ("s SATISFIABLE", "v ", "s UNSATISFIABLE")
+    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()
+    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()
+    val result     = SatSolver.make_external_solver cmd writefn readfn fm
+    val _          = try File.rm inpath
+    val _          = try File.rm outpath
+  in
+    result
+  end
 in
-	SatSolver.add_solver ("jerusat", jerusat)
+  SatSolver.add_solver ("jerusat", jerusat)
 end;
 
 (* ------------------------------------------------------------------------- *)
 (* Add code for other SAT solvers below.                                     *)
 (* ------------------------------------------------------------------------- *)
 
 (*
 let
-	fun mysolver fm = ...
+  fun mysolver fm = ...
 in
-	SatSolver.add_solver ("myname", (fn fm => if mysolver_is_configured then mysolver fm else raise SatSolver.NOT_CONFIGURED));  -- register the solver
+  SatSolver.add_solver ("myname", (fn fm => if mysolver_is_configured then mysolver fm else raise SatSolver.NOT_CONFIGURED));  -- register the solver
 end;
 
 -- the solver is now available as SatSolver.invoke_solver "myname"
 *)