src/Pure/unify.ML

discontinue export_document --- always enabled (reverting f0f83ce0badd);

(*  Title:      Pure/unify.ML
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   Cambridge University 1992

Higher-Order Unification.

Types as well as terms are unified.  The outermost functions assume
the terms to be unified already have the same type.  In resolution,
this is assured because both have type "prop".
*)

signature UNIFY =
sig
  val trace_bound: int Config.T
  val search_bound: int Config.T
  val trace_simp: bool Config.T
  val trace_types: bool Config.T
  val hounifiers: Context.generic * Envir.env * ((term * term) list) ->
    (Envir.env * (term * term) list) Seq.seq
  val unifiers: Context.generic * Envir.env * ((term * term) list) ->
    (Envir.env * (term * term) list) Seq.seq
  val smash_unifiers: Context.generic -> (term * term) list -> Envir.env -> Envir.env Seq.seq
end

structure Unify : UNIFY =
struct

(*Unification options*)

(*tracing starts above this depth, 0 for full*)
val trace_bound = Config.declare_int ("unify_trace_bound", \<^here>) (K 50);

(*unification quits above this depth*)
val search_bound = Config.declare_int ("unify_search_bound", \<^here>) (K 60);

(*print dpairs before calling SIMPL*)
val trace_simp = Config.declare_bool ("unify_trace_simp", \<^here>) (K false);

(*announce potential incompleteness of type unification*)
val trace_types = Config.declare_bool ("unify_trace_types", \<^here>) (K false);


type binderlist = (string * typ) list;

type dpair = binderlist * term * term;

fun fastype env (Ts, t) = Envir.fastype env (map snd Ts) t;


(* eta normal form *)

fun eta_norm env =
  let
    val tyenv = Envir.type_env env;
    fun etif (Type ("fun", [T, U]), t) =
          Abs ("", T, etif (U, incr_boundvars 1 t $ Bound 0))
      | etif (TVar v, t) =
          (case Type.lookup tyenv v of
            NONE => t
          | SOME T => etif (T, t))
      | etif (_, t) = t;
    fun eta_nm (rbinder, Abs (a, T, body)) =
          Abs (a, T, eta_nm ((a, T) :: rbinder, body))
      | eta_nm (rbinder, t) = etif (fastype env (rbinder, t), t);
  in eta_nm end;


(*OCCURS CHECK
  Does the uvar occur in the term t?
  two forms of search, for whether there is a rigid path to the current term.
  "seen" is list of variables passed thru, is a memo variable for sharing.
  This version searches for nonrigid occurrence, returns true if found.
  Since terms may contain variables with same name and different types,
  the occurs check must ignore the types of variables. This avoids
  that ?x::?'a is unified with f(?x::T), which may lead to a cyclic
  substitution when ?'a is instantiated with T later. *)
fun occurs_terms (seen: indexname list Unsynchronized.ref,
      env: Envir.env, v: indexname, ts: term list): bool =
  let
    fun occurs [] = false
      | occurs (t :: ts) = occur t orelse occurs ts
    and occur (Const _) = false
      | occur (Bound _) = false
      | occur (Free _) = false
      | occur (Var (w, T)) =
          if member (op =) (!seen) w then false
          else if Term.eq_ix (v, w) then true
            (*no need to lookup: v has no assignment*)
          else
            (seen := w :: !seen;
             case Envir.lookup env (w, T) of
               NONE => false
             | SOME t => occur t)
      | occur (Abs (_, _, body)) = occur body
      | occur (f $ t) = occur t orelse occur f;
  in occurs ts end;


(* f a1 ... an  ---->  f  using the assignments*)
fun head_of_in env t =
  (case t of
    f $ _ => head_of_in env f
  | Var vT =>
      (case Envir.lookup env vT of
        SOME u => head_of_in env u
      | NONE => t)
  | _ => t);


datatype occ = NoOcc | Nonrigid | Rigid;

(* Rigid occur check
Returns Rigid    if it finds a rigid occurrence of the variable,
        Nonrigid if it finds a nonrigid path to the variable.
        NoOcc    otherwise.
  Continues searching for a rigid occurrence even if it finds a nonrigid one.

Condition for detecting non-unifable terms: [ section 5.3 of Huet (1975) ]
   a rigid path to the variable, appearing with no arguments.
Here completeness is sacrificed in order to reduce danger of divergence:
   reject ALL rigid paths to the variable.
Could check for rigid paths to bound variables that are out of scope.
Not necessary because the assignment test looks at variable's ENTIRE rbinder.

Treatment of head(arg1,...,argn):
If head is a variable then no rigid path, switch to nonrigid search
for arg1,...,argn.
If head is an abstraction then possibly no rigid path (head could be a
   constant function) so again use nonrigid search.  Happens only if
   term is not in normal form.

Warning: finds a rigid occurrence of ?f in ?f(t).
  Should NOT be called in this case: there is a flex-flex unifier
*)
fun rigid_occurs_term (seen: indexname list Unsynchronized.ref, env, v: indexname, t) =
  let
    fun nonrigid t =
      if occurs_terms (seen, env, v, [t]) then Nonrigid
      else NoOcc
    fun occurs [] = NoOcc
      | occurs (t :: ts) =
          (case occur t of
            Rigid => Rigid
          | oc => (case occurs ts of NoOcc => oc | oc2 => oc2))
    and occomb (f $ t) =
        (case occur t of
          Rigid => Rigid
        | oc => (case occomb f of NoOcc => oc | oc2 => oc2))
      | occomb t = occur t
    and occur (Const _) = NoOcc
      | occur (Bound _) = NoOcc
      | occur (Free _) = NoOcc
      | occur (Var (w, T)) =
          if member (op =) (!seen) w then NoOcc
          else if Term.eq_ix (v, w) then Rigid
          else
            (seen := w :: !seen;
             case Envir.lookup env (w, T) of
               NONE => NoOcc
             | SOME t => occur t)
      | occur (Abs (_, _, body)) = occur body
      | occur (t as f $ _) =  (*switch to nonrigid search?*)
          (case head_of_in env f of
            Var (w,_) => (*w is not assigned*)
              if Term.eq_ix (v, w) then Rigid
              else nonrigid t
          | Abs _ => nonrigid t (*not in normal form*)
          | _ => occomb t)
  in occur t end;


exception CANTUNIFY;  (*Signals non-unifiability.  Does not signal errors!*)
exception ASSIGN;  (*Raised if not an assignment*)


fun unify_types context TU env =
  Pattern.unify_types context TU env handle Pattern.Unif => raise CANTUNIFY;

fun test_unify_types context (T, U) env =
  let
    fun trace () =
      if Context_Position.is_visible_generic context then
        let val str_of = Syntax.string_of_typ (Context.proof_of context)
        in tracing ("Potential loss of completeness: " ^ str_of U ^ " = " ^ str_of T) end
      else ();
    val env' = unify_types context (T, U) env;
  in if is_TVar T orelse is_TVar U then trace () else (); env' end;

(*Is the term eta-convertible to a single variable with the given rbinder?
  Examples: ?a   ?f(B.0)   ?g(B.1,B.0)
  Result is var a for use in SIMPL. *)
fun get_eta_var ([], _, Var vT) = vT
  | get_eta_var (_::rbinder, n, f $ Bound i) =
      if n = i then get_eta_var (rbinder, n + 1, f)
      else raise ASSIGN
  | get_eta_var _ = raise ASSIGN;


(*Solve v=u by assignment -- "fixedpoint" to Huet -- if v not in u.
  If v occurs rigidly then nonunifiable.
  If v occurs nonrigidly then must use full algorithm. *)
fun assignment context (rbinder, t, u) env =
  let val vT as (v,T) = get_eta_var (rbinder, 0, t) in
    (case rigid_occurs_term (Unsynchronized.ref [], env, v, u) of
      NoOcc =>
        let val env = unify_types context (Envir.body_type env T, fastype env (rbinder, u)) env
        in Envir.update (vT, Logic.rlist_abs (rbinder, u)) env end
    | Nonrigid => raise ASSIGN
    | Rigid => raise CANTUNIFY)
  end;


(*Extends an rbinder with a new disagreement pair, if both are abstractions.
  Tries to unify types of the bound variables!
  Checks that binders have same length, since terms should be eta-normal;
    if not, raises TERM, probably indicating type mismatch.
  Uses variable a (unless the null string) to preserve user's naming.*)
fun new_dpair context (rbinder, Abs (a, T, body1), Abs (b, U, body2)) env =
      let
        val env' = unify_types context (T, U) env;
        val c = if a = "" then b else a;
      in new_dpair context ((c,T) :: rbinder, body1, body2) env' end
  | new_dpair _ (_, Abs _, _) _ = raise TERM ("new_dpair", [])
  | new_dpair _ (_, _, Abs _) _ = raise TERM ("new_dpair", [])
  | new_dpair _ (rbinder, t1, t2) env = ((rbinder, t1, t2), env);


fun head_norm_dpair context (env, (rbinder, t, u)) : dpair * Envir.env =
  new_dpair context (rbinder,
    eta_norm env (rbinder, Envir.head_norm env t),
    eta_norm env (rbinder, Envir.head_norm env u)) env;



(*flexflex: the flex-flex pairs,  flexrigid: the flex-rigid pairs
  Does not perform assignments for flex-flex pairs:
    may create nonrigid paths, which prevent other assignments.
  Does not even identify Vars in dpairs such as ?a \<equiv>\<^sup>? ?b; an attempt to
    do so caused numerous problems with no compensating advantage.
*)
fun SIMPL0 context dp0 (env,flexflex,flexrigid) : Envir.env * dpair list * dpair list =
  let
    val (dp as (rbinder, t, u), env) = head_norm_dpair context (env, dp0);
    fun SIMRANDS (f $ t, g $ u, env) =
          SIMPL0 context (rbinder, t, u) (SIMRANDS (f, g, env))
      | SIMRANDS (t as _$_, _, _) =
          raise TERM ("SIMPL: operands mismatch", [t, u])
      | SIMRANDS (t, u as _ $ _, _) =
          raise TERM ("SIMPL: operands mismatch", [t, u])
      | SIMRANDS (_, _, env) = (env, flexflex, flexrigid);
  in
    (case (head_of t, head_of u) of
      (Var (_, T), Var (_, U)) =>
        let
          val T' = Envir.body_type env T and U' = Envir.body_type env U;
          val env = unify_types context (T', U') env;
        in (env, dp :: flexflex, flexrigid) end
    | (Var _, _) =>
        ((assignment context (rbinder,t,u) env, flexflex, flexrigid)
          handle ASSIGN => (env, flexflex, dp :: flexrigid))
    | (_, Var _) =>
        ((assignment context (rbinder, u, t) env, flexflex, flexrigid)
          handle ASSIGN => (env, flexflex, (rbinder, u, t) :: flexrigid))
    | (Const (a, T), Const (b, U)) =>
        if a = b then SIMRANDS (t, u, unify_types context (T, U) env)
        else raise CANTUNIFY
    | (Bound i, Bound j) =>
        if i = j then SIMRANDS (t, u, env) else raise CANTUNIFY
    | (Free (a, T), Free (b, U)) =>
        if a = b then SIMRANDS (t, u, unify_types context (T, U) env)
        else raise CANTUNIFY
    | _ => raise CANTUNIFY)
  end;


(* changed(env,t) checks whether the head of t is a variable assigned in env*)
fun changed env (f $ _) = changed env f
  | changed env (Var v) = (case Envir.lookup env v of NONE => false | _ => true)
  | changed _ _ = false;


(*Recursion needed if any of the 'head variables' have been updated
  Clever would be to re-do just the affected dpairs*)
fun SIMPL context (env,dpairs) : Envir.env * dpair list * dpair list =
  let
    val all as (env', flexflex, flexrigid) = fold_rev (SIMPL0 context) dpairs (env, [], []);
    val dps = flexrigid @ flexflex;
  in
    if exists (fn (_, t, u) => changed env' t orelse changed env' u) dps
    then SIMPL context (env', dps) else all
  end;


(*Makes the terms E1,...,Em,    where Ts = [T...Tm].
  Each Ei is   ?Gi(B.(n-1),...,B.0), and has type Ti
  The B.j are bound vars of binder.
  The terms are not made in eta-normal-form, SIMPL does that later.
  If done here, eta-expansion must be recursive in the arguments! *)
fun make_args _ (_, env, []) = (env, [])   (*frequent case*)
  | make_args name (binder: typ list, env, Ts) : Envir.env * term list =
      let
        fun funtype T = binder ---> T;
        val (env', vars) = Envir.genvars name (env, map funtype Ts);
      in (env', map (fn var => Logic.combound (var, 0, length binder)) vars) end;


(*Abstraction over a list of types*)
fun types_abs ([], u) = u
  | types_abs (T :: Ts, u) = Abs ("", T, types_abs (Ts, u));

(*Abstraction over the binder of a type*)
fun type_abs (env, T, t) = types_abs (Envir.binder_types env T, t);


(*MATCH taking "big steps".
  Copies u into the Var v, using projection on targs or imitation.
  A projection is allowed unless SIMPL raises an exception.
  Allocates new variables in projection on a higher-order argument,
    or if u is a variable (flex-flex dpair).
  Returns long sequence of every way of copying u, for backtracking
  For example, projection in ?b'(?a) may be wrong if other dpairs constrain ?a.
  The order for trying projections is crucial in ?b'(?a)
  NB "vname" is only used in the call to make_args!!   *)
fun matchcopy context vname =
  let
    fun mc (rbinder, targs, u, ed as (env, dpairs)) : (term * (Envir.env * dpair list)) Seq.seq =
      let
        val trace_types = Config.get_generic context trace_types;
        (*Produce copies of uarg and cons them in front of uargs*)
        fun copycons uarg (uargs, (env, dpairs)) =
          Seq.map (fn (uarg', ed') => (uarg' :: uargs, ed'))
            (mc (rbinder, targs,eta_norm env (rbinder, Envir.head_norm env uarg),
              (env, dpairs)));
        (*Produce sequence of all possible ways of copying the arg list*)
        fun copyargs [] = Seq.cons ([], ed) Seq.empty
          | copyargs (uarg :: uargs) = Seq.maps (copycons uarg) (copyargs uargs);
        val (uhead, uargs) = strip_comb u;
        val base = Envir.body_type env (fastype env (rbinder, uhead));
        fun joinargs (uargs', ed') = (list_comb (uhead, uargs'), ed');
        (*attempt projection on argument with given typ*)
        val Ts = map (curry (fastype env) rbinder) targs;
        fun projenv (head, (Us, bary), targ, tail) =
          let
            val env =
              if trace_types then test_unify_types context (base, bary) env
              else unify_types context (base, bary) env
          in
            Seq.make (fn () =>
              let
                val (env', args) = make_args vname (Ts, env, Us);
                (*higher-order projection: plug in targs for bound vars*)
                fun plugin arg = list_comb (head_of arg, targs);
                val dp = (rbinder, list_comb (targ, map plugin args), u);
                val (env2, frigid, fflex) = SIMPL context (env', dp :: dpairs);
                (*may raise exception CANTUNIFY*)
              in
                SOME ((list_comb (head, args), (env2, frigid @ fflex)), tail)
              end handle CANTUNIFY => Seq.pull tail)
          end handle CANTUNIFY => tail;
        (*make a list of projections*)
        fun make_projs (T::Ts, targ::targs) =
            (Bound(length Ts), T, targ) :: make_projs (Ts,targs)
          | make_projs ([],[]) = []
          | make_projs _ = raise TERM ("make_projs", u::targs);
        (*try projections and imitation*)
        fun matchfun ((bvar,T,targ)::projs) =
             (projenv(bvar, Envir.strip_type env T, targ, matchfun projs))
          | matchfun [] = (*imitation last of all*)
            (case uhead of
         Const _ => Seq.map joinargs (copyargs uargs)
             | Free _  => Seq.map joinargs (copyargs uargs)
             | _ => Seq.empty)  (*if Var, would be a loop!*)
    in
      (case uhead of
        Abs (a, T, body) =>
          Seq.map (fn (body', ed') => (Abs (a, T, body'), ed'))
            (mc ((a, T) :: rbinder, (map (incr_boundvars 1) targs) @ [Bound 0], body, ed))
      | Var (w, _) =>
          (*a flex-flex dpair: make variable for t*)
          let
            val (env', newhd) = Envir.genvar (#1 w) (env, Ts ---> base);
            val tabs = Logic.combound (newhd, 0, length Ts);
            val tsub = list_comb (newhd, targs);
          in Seq.single (tabs, (env', (rbinder, tsub, u) :: dpairs)) end
      | _ => matchfun (rev (make_projs (Ts, targs))))
    end;
  in mc end;


(*Call matchcopy to produce assignments to the variable in the dpair*)
fun MATCH context (env, (rbinder, t, u), dpairs) : (Envir.env * dpair list) Seq.seq =
  let
    val (Var (vT as (v, T)), targs) = strip_comb t;
    val Ts = Envir.binder_types env T;
    fun new_dset (u', (env', dpairs')) =
      (*if v was updated to s, must unify s with u' *)
      (case Envir.lookup env' vT of
        NONE => (Envir.update (vT, types_abs (Ts, u')) env', dpairs')
      | SOME s => (env', ([], s, types_abs (Ts, u')) :: dpairs'));
  in
    Seq.map new_dset (matchcopy context (#1 v) (rbinder, targs, u, (env, dpairs)))
  end;



(**** Flex-flex processing ****)

(*At end of unification, do flex-flex assignments like ?a -> ?f(?b)
  Attempts to update t with u, raising ASSIGN if impossible*)
fun ff_assign context (env, rbinder, t, u) : Envir.env =
  let val vT as (v, T) = get_eta_var (rbinder, 0, t) in
    if occurs_terms (Unsynchronized.ref [], env, v, [u]) then raise ASSIGN
    else
      let val env = unify_types context (Envir.body_type env T, fastype env (rbinder, u)) env
      in Envir.vupdate (vT, Logic.rlist_abs (rbinder, u)) env end
  end;


(*If an argument contains a banned Bound, then it should be deleted.
  But if the only path is flexible, this is difficult; the code gives up!
  In \<lambda>x y. ?a x \<equiv>\<^sup>? \<lambda>x y. ?b (?c y) should we instantiate ?b or ?c *)
exception CHANGE_FAIL;   (*flexible occurrence of banned variable, or other reason to quit*)


(*Flex argument: a term, its type, and the index that refers to it.*)
type flarg = {t: term, T: typ, j: int};

(*Form the arguments into records for deletion/sorting.*)
fun flexargs ([], [], []) = [] : flarg list
  | flexargs (j :: js, t :: ts, T :: Ts) = {j = j, t = t, T = T} :: flexargs (js, ts, Ts)
  | flexargs _ = raise CHANGE_FAIL;
(*We give up if we see a variable of function type not applied to a full list of
  arguments (remember, this code assumes that terms are fully eta-expanded).  This situation
  can occur if a type variable is instantiated with a function type.
*)

(*Check whether the 'banned' bound var indices occur rigidly in t*)
fun rigid_bound (lev, banned) t =
  let val (head,args) = strip_comb t in
    (case head of
      Bound i =>
        member (op =) banned (i - lev) orelse exists (rigid_bound (lev, banned)) args
    | Var _ => false  (*no rigid occurrences here!*)
    | Abs (_, _, u) =>
        rigid_bound (lev + 1, banned) u orelse
        exists (rigid_bound (lev, banned)) args
    | _ => exists (rigid_bound (lev, banned)) args)
  end;

(*Squash down indices at level >=lev to delete the banned from a term.*)
fun change_bnos banned =
  let
    fun change lev (Bound i) =
          if i < lev then Bound i
          else if member (op =) banned (i - lev) then
            raise CHANGE_FAIL (**flexible occurrence: give up**)
          else Bound (i - length (filter (fn j => j < i - lev) banned))
      | change lev (Abs (a, T, t)) = Abs (a, T, change(lev + 1) t)
      | change lev (t $ u) = change lev t $ change lev u
      | change lev t = t;
  in change 0 end;

(*Change indices, delete the argument if it contains a banned Bound*)
fun change_arg banned {j, t, T} args : flarg list =
  if rigid_bound (0, banned) t then args  (*delete argument!*)
  else {j = j, t = change_bnos banned t, T = T} :: args;


(*Sort the arguments to create assignments if possible:
  create eta-terms like ?g B.1 B.0*)
local
  fun less_arg ({t = Bound i1, ...}, {t = Bound i2, ...}) = (i2 < i1)
    | less_arg (_: flarg, _: flarg) = false;

  fun ins_arg x [] = [x]
    | ins_arg x (y :: ys) =
        if less_arg (y, x) then y :: ins_arg x ys else x :: y :: ys;
in
  fun sort_args [] = []
    | sort_args (x :: xs) = ins_arg x (sort_args xs);
end;

(*Test whether the new term would be eta-equivalent to a variable --
  if so then there is no point in creating a new variable*)
fun decreasing n ([]: flarg list) = (n = 0)
  | decreasing n ({j, ...} :: args) = j = n - 1 andalso decreasing (n - 1) args;

(*Delete banned indices in the term, simplifying it.
  Force an assignment, if possible, by sorting the arguments.
  Update its head; squash indices in arguments. *)
fun clean_term banned (env,t) =
  let
    val (Var (v, T), ts) = strip_comb t;
    val (Ts, U) = Envir.strip_type env T
    and js = length ts - 1  downto 0;
    val args = sort_args (fold_rev (change_arg banned) (flexargs (js, ts, Ts)) [])
    val ts' = map #t args;
  in
    if decreasing (length Ts) args then (env, (list_comb (Var (v, T), ts')))
    else
      let
        val (env', v') = Envir.genvar (#1 v) (env, map #T args ---> U);
        val body = list_comb (v', map (Bound o #j) args);
        val env2 = Envir.vupdate ((v, T), types_abs (Ts, body)) env';
        (*the vupdate affects ts' if they contain v*)
      in (env2, Envir.norm_term env2 (list_comb (v', ts'))) end
  end;


(*Add tpair if not trivial or already there.
  Should check for swapped pairs??*)
fun add_tpair (rbinder, (t0, u0), tpairs) : (term * term) list =
  if t0 aconv u0 then tpairs
  else
    let
      val t = Logic.rlist_abs (rbinder, t0)
      and u = Logic.rlist_abs (rbinder, u0);
      fun same (t', u') = (t aconv t') andalso (u aconv u')
    in if exists same tpairs then tpairs else (t, u) :: tpairs end;


(*Simplify both terms and check for assignments.
  Bound vars in the binder are "banned" unless used in both t AND u *)
fun clean_ffpair context ((rbinder, t, u), (env, tpairs)) =
  let
    val loot = loose_bnos t and loou = loose_bnos u
    fun add_index (j, (a, T)) (bnos, newbinder) =
      if member (op =) loot j andalso member (op =) loou j
      then (bnos, (a, T) :: newbinder)  (*needed by both: keep*)
      else (j :: bnos, newbinder);   (*remove*)
    val (banned, rbin') = fold_rev add_index ((0 upto (length rbinder - 1)) ~~ rbinder) ([], []);
    val (env', t') = clean_term banned (env, t);
    val (env'',u') = clean_term banned (env',u);
  in
    (ff_assign context (env'', rbin', t', u'), tpairs)
      handle ASSIGN =>
        (ff_assign context (env'', rbin', u', t'), tpairs)
          handle ASSIGN => (env'', add_tpair (rbin', (t', u'), tpairs))
  end
  handle CHANGE_FAIL => (env, add_tpair (rbinder, (t, u), tpairs));


(*IF the flex-flex dpair is an assignment THEN do it  ELSE  put in tpairs
  eliminates trivial tpairs like t=t, as well as repeated ones
  trivial tpairs can easily escape SIMPL:  ?A=t, ?A=?B, ?B=t gives t=t
  Resulting tpairs MAY NOT be in normal form:  assignments may occur here.*)
fun add_ffpair context (rbinder,t0,u0) (env,tpairs) : Envir.env * (term * term) list =
  let
    val t = Envir.norm_term env t0
    and u = Envir.norm_term env u0;
  in
    (case (head_of t, head_of u) of
      (Var (v, T), Var (w, U)) =>  (*Check for identical variables...*)
        if Term.eq_ix (v, w) then     (*...occur check would falsely return true!*)
          if T = U then (env, add_tpair (rbinder, (t, u), tpairs))
          else raise TERM ("add_ffpair: Var name confusion", [t, u])
        else if Term_Ord.indexname_ord (v, w) = LESS then (*prefer to update the LARGER variable*)
          clean_ffpair context ((rbinder, u, t), (env, tpairs))
        else clean_ffpair context ((rbinder, t, u), (env, tpairs))
    | _ => raise TERM ("add_ffpair: Vars expected", [t, u]))
  end;


(*Print a tracing message + list of dpairs.
  In t \<equiv> u print u first because it may be rigid or flexible --
    t is always flexible.*)
fun print_dpairs context msg (env, dpairs) =
  if Context_Position.is_visible_generic context then
    let
      fun pdp (rbinder, t, u) =
        let
          val ctxt = Context.proof_of context;
          fun termT t =
            Syntax.pretty_term ctxt (Envir.norm_term env (Logic.rlist_abs (rbinder, t)));
          val prt = Pretty.blk (0, [termT u, Pretty.str " \<equiv>\<^sup>?", Pretty.brk 1, termT t]);
        in tracing (Pretty.string_of prt) end;
    in tracing msg; List.app pdp dpairs end
  else ();


(*Unify the dpairs in the environment.
  Returns flex-flex disagreement pairs NOT IN normal form.
  SIMPL may raise exception CANTUNIFY. *)
fun hounifiers (context, env, tus : (term * term) list) : (Envir.env * (term * term) list) Seq.seq =
  let
    val trace_bound = Config.get_generic context trace_bound;
    val search_bound = Config.get_generic context search_bound;
    val trace_simp = Config.get_generic context trace_simp;
    fun add_unify tdepth ((env, dpairs), reseq) =
      Seq.make (fn () =>
        let
          val (env', flexflex, flexrigid) =
           (if tdepth > trace_bound andalso trace_simp
            then print_dpairs context "Enter SIMPL" (env, dpairs) else ();
            SIMPL context (env, dpairs));
        in
          (case flexrigid of
            [] => SOME (fold_rev (add_ffpair context) flexflex (env', []), reseq)
          | dp :: frigid' =>
              if tdepth > search_bound then
                (if Context_Position.is_visible_generic context
                 then warning "Unification bound exceeded" else (); Seq.pull reseq)
              else
               (if tdepth > trace_bound then
                  print_dpairs context "Enter MATCH" (env',flexrigid@flexflex)
                else ();
                Seq.pull (Seq.it_right
                    (add_unify (tdepth + 1)) (MATCH context (env',dp, frigid'@flexflex), reseq))))
        end
        handle CANTUNIFY =>
         (if tdepth > trace_bound andalso Context_Position.is_visible_generic context
          then tracing "Failure node"
          else (); Seq.pull reseq));
    val dps = map (fn (t, u) => ([], t, u)) tus;
  in add_unify 1 ((env, dps), Seq.empty) end;

fun unifiers (params as (context, env, tus)) =
  Seq.cons (fold (Pattern.unify context) tus env, []) Seq.empty
    handle Pattern.Unif => Seq.empty
      | Pattern.Pattern => hounifiers params;


(*For smash_flexflex1*)
fun var_head_of (env,t) : indexname * typ =
  (case head_of (strip_abs_body (Envir.norm_term env t)) of
    Var (v, T) => (v, T)
  | _ => raise CANTUNIFY);  (*not flexible, cannot use trivial substitution*)


(*Eliminate a flex-flex pair by the trivial substitution, see Huet (1975)
  Unifies ?f t1 ... rm with ?g u1 ... un by ?f -> \<lambda>x1...xm. ?a, ?g -> \<lambda>x1...xn. ?a
  Unfortunately, unifies ?f t u with ?g t u by ?f, ?g -> \<lambda>x y. ?a,
  though just ?g->?f is a more general unifier.
  Unlike Huet (1975), does not smash together all variables of same type --
    requires more work yet gives a less general unifier (fewer variables).
  Handles ?f t1 ... rm with ?f u1 ... um to avoid multiple updates. *)
fun smash_flexflex1 (t, u) env : Envir.env =
  let
    val vT as (v, T) = var_head_of (env, t)
    and wU as (w, U) = var_head_of (env, u);
    val (env', var) = Envir.genvar (#1 v) (env, Envir.body_type env T);
    val env'' = Envir.vupdate (wU, type_abs (env', U, var)) env';
  in
    if vT = wU then env''  (*the other update would be identical*)
    else Envir.vupdate (vT, type_abs (env', T, var)) env''
  end;


(*Smash all flex-flexpairs.  Should allow selection of pairs by a predicate?*)
fun smash_flexflex (env, tpairs) : Envir.env =
  fold_rev smash_flexflex1 tpairs env;

(*Returns unifiers with no remaining disagreement pairs*)
fun smash_unifiers context tus env =
  Seq.map smash_flexflex (unifiers (context, env, tus));

end;