author  nipkow 
Thu, 21 Dec 2000 16:18:40 +0100  
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parent 10691  4ea37fba9c02 
child 12109  bd6eb9194a5d 
permissions  rwrr 
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(* Title: Provers/Arith/fast_lin_arith.ML 
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ID: $Id$ 
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Author: Tobias Nipkow 
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Copyright 1998 TU Munich 
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6062  6 
A generic linear arithmetic package. 
6102  7 
It provides two tactics 
8 

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lin_arith_tac: int > tactic 
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cut_lin_arith_tac: thms > int > tactic 
6102  11 

12 
and a simplification procedure 

13 

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lin_arith_prover: Sign.sg > thm list > term > thm option 

15 

16 
Only take premises and conclusions into account that are already (negated) 

17 
(in)equations. lin_arith_prover tries to prove or disprove the term. 

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*) 
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9073  20 
(* Debugging: set Fast_Arith.trace *) 
7582  21 

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(*** Data needed for setting up the linear arithmetic package ***) 
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6102  24 
signature LIN_ARITH_LOGIC = 
25 
sig 

26 
val conjI: thm 

27 
val ccontr: thm (* (~ P ==> False) ==> P *) 

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val neqE: thm (* [ m ~= n; m < n ==> P; n < m ==> P ] ==> P *) 

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val notI: thm (* (P ==> False) ==> ~ P *) 

6110  30 
val not_lessD: thm (* ~(m < n) ==> n <= m *) 
6128  31 
val not_leD: thm (* ~(m <= n) ==> n < m *) 
6102  32 
val sym: thm (* x = y ==> y = x *) 
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val mk_Eq: thm > thm 

34 
val mk_Trueprop: term > term 

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val neg_prop: term > term 

36 
val is_False: thm > bool 

6128  37 
val is_nat: typ list * term > bool 
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val mk_nat_thm: Sign.sg > term > thm 

6102  39 
end; 
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(* 

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mk_Eq(~in) = `in == False' 

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mk_Eq(in) = `in == True' 

43 
where `in' is an (in)equality. 

44 

45 
neg_prop(t) = neg if t is wrapped up in Trueprop and 

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nt is the (logically) negated version of t, where the negation 

47 
of a negative term is the term itself (no double negation!); 

6128  48 

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is_nat(parametertypes,t) = t:nat 

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mk_nat_thm(t) = "0 <= t" 

6102  51 
*) 
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signature LIN_ARITH_DATA = 
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sig 
6128  55 
val decomp: 
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Sign.sg > term > ((term*rat)list * rat * string * (term*rat)list * rat * bool)option 
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val number_of: int * typ > term 

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end; 
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(* 
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decomp(`x Rel y') should yield (p,i,Rel,q,j,d) 
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where Rel is one of "<", "~<", "<=", "~<=" and "=" and 
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p/q is the decomposition of the sum terms x/y into a list 
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of summand * multiplicity pairs and a constant summand and 
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d indicates if the domain is discrete. 
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9420  66 
ss must reduce contradictory <= to False. 
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It should also cancel common summands to keep <= reduced; 
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otherwise <= can grow to massive proportions. 
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*) 
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6062  71 
signature FAST_LIN_ARITH = 
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sig 

9420  73 
val setup: (theory > theory) list 
10691  74 
val map_data: ({add_mono_thms: thm list, mult_mono_thms: (thm*cterm)list, inj_thms: thm list, 
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lessD: thm list, simpset: Simplifier.simpset} 
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> {add_mono_thms: thm list, mult_mono_thms: (thm*cterm)list, inj_thms: thm list, 
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lessD: thm list, simpset: Simplifier.simpset}) 
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> theory > theory 

9073  79 
val trace : bool ref 
6074  80 
val lin_arith_prover: Sign.sg > thm list > term > thm option 
6062  81 
val lin_arith_tac: int > tactic 
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val cut_lin_arith_tac: thm list > int > tactic 

83 
end; 

84 

6102  85 
functor Fast_Lin_Arith(structure LA_Logic:LIN_ARITH_LOGIC 
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and LA_Data:LIN_ARITH_DATA) : FAST_LIN_ARITH = 

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struct 
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9420  89 

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(** theory data **) 

91 

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(* data kind 'Provers/fast_lin_arith' *) 

93 

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structure DataArgs = 

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struct 

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val name = "Provers/fast_lin_arith"; 

10691  97 
type T = {add_mono_thms: thm list, mult_mono_thms: (thm*cterm)list, inj_thms: thm list, 
10575  98 
lessD: thm list, simpset: Simplifier.simpset}; 
9420  99 

10691  100 
val empty = {add_mono_thms = [], mult_mono_thms = [], inj_thms = [], 
10575  101 
lessD = [], simpset = Simplifier.empty_ss}; 
9420  102 
val copy = I; 
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val prep_ext = I; 

104 

10691  105 
fun merge ({add_mono_thms= add_mono_thms1, mult_mono_thms= mult_mono_thms1, inj_thms= inj_thms1, 
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lessD = lessD1, simpset = simpset1}, 
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{add_mono_thms= add_mono_thms2, mult_mono_thms= mult_mono_thms2, inj_thms= inj_thms2, 
10575  108 
lessD = lessD2, simpset = simpset2}) = 
9420  109 
{add_mono_thms = Drule.merge_rules (add_mono_thms1, add_mono_thms2), 
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mult_mono_thms= generic_merge (eq_thm o pairself fst) I I mult_mono_thms1 mult_mono_thms2, 
10575  111 
inj_thms = Drule.merge_rules (inj_thms1, inj_thms2), 
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lessD = Drule.merge_rules (lessD1, lessD2), 

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simpset = Simplifier.merge_ss (simpset1, simpset2)}; 

9420  114 

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fun print _ _ = (); 

116 
end; 

117 

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structure Data = TheoryDataFun(DataArgs); 

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val map_data = Data.map; 

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val setup = [Data.init]; 

121 

122 

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(*** A fast decision procedure ***) 
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(*** Code ported from HOL Light ***) 
6056  126 
(* possible optimizations: 
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use (var,coeff) rep or vector rep tp save space; 

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treat nonnegative atoms separately rather than adding 0 <= atom 

129 
*) 

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9073  131 
val trace = ref false; 
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datatype lineq_type = Eq  Le  Lt; 
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6056  135 
datatype injust = Asm of int 
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 Nat of int (* index of atom *) 

6128  137 
 LessD of injust 
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 NotLessD of injust 

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 NotLeD of injust 

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 NotLeDD of injust 
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 Multiplied of int * injust 
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 Multiplied2 of int * injust 
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 Added of injust * injust; 
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datatype lineq = Lineq of int * lineq_type * int list * injust; 
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(*  *) 
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(* Calculate new (in)equality type after addition. *) 
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(*  *) 
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fun find_add_type(Eq,x) = x 
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 find_add_type(x,Eq) = x 
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 find_add_type(_,Lt) = Lt 
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 find_add_type(Lt,_) = Lt 
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 find_add_type(Le,Le) = Le; 
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(*  *) 
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(* Multiply out an (in)equation. *) 
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(*  *) 
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fun multiply_ineq n (i as Lineq(k,ty,l,just)) = 
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if n = 1 then i 
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else if n = 0 andalso ty = Lt then sys_error "multiply_ineq" 
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else if n < 0 andalso (ty=Le orelse ty=Lt) then sys_error "multiply_ineq" 
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else Lineq(n * k,ty,map (apl(n,op * )) l,Multiplied(n,just)); 
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(*  *) 
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(* Add together (in)equations. *) 
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(*  *) 
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fun add_ineq (i1 as Lineq(k1,ty1,l1,just1)) (i2 as Lineq(k2,ty2,l2,just2)) = 
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let val l = map2 (op +) (l1,l2) 
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in Lineq(k1+k2,find_add_type(ty1,ty2),l,Added(just1,just2)) end; 
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(*  *) 
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(* Elimination of variable between a single pair of (in)equations. *) 
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(* If they're both inequalities, 1st coefficient must be +ve, 2nd ve. *) 
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(*  *) 
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fun el 0 (h::_) = h 
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 el n (_::t) = el (n  1) t 
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 el _ _ = sys_error "el"; 
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fun elim_var v (i1 as Lineq(k1,ty1,l1,just1)) (i2 as Lineq(k2,ty2,l2,just2)) = 
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let val c1 = el v l1 and c2 = el v l2 
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val m = lcm(abs c1,abs c2) 
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val m1 = m div (abs c1) and m2 = m div (abs c2) 
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val (n1,n2) = 
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if (c1 >= 0) = (c2 >= 0) 
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then if ty1 = Eq then (~m1,m2) 
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else if ty2 = Eq then (m1,~m2) 
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else sys_error "elim_var" 
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else (m1,m2) 
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val (p1,p2) = if ty1=Eq andalso ty2=Eq andalso (n1 = ~1 orelse n2 = ~1) 
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then (~n1,~n2) else (n1,n2) 
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in add_ineq (multiply_ineq n1 i1) (multiply_ineq n2 i2) end; 
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(*  *) 
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(* The main refutationfinding code. *) 
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(*  *) 
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fun is_trivial (Lineq(_,_,l,_)) = forall (fn i => i=0) l; 
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fun is_answer (ans as Lineq(k,ty,l,_)) = 
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case ty of Eq => k <> 0  Le => k > 0  Lt => k >= 0; 
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fun calc_blowup l = 
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let val (p,n) = partition (apl(0,op<)) (filter (apl(0,op<>)) l) 
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in (length p) * (length n) end; 
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210 

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(*  *) 
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(* Main elimination code: *) 
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(* *) 
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(* (1) Looks for immediate solutions (false assertions with no variables). *) 
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(* *) 
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(* (2) If there are any equations, picks a variable with the lowest absolute *) 
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(* coefficient in any of them, and uses it to eliminate. *) 
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(* *) 
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(* (3) Otherwise, chooses a variable in the inequality to minimize the *) 
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(* blowup (number of consequences generated) and eliminates it. *) 
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(*  *) 
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222 

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fun allpairs f xs ys = 
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flat(map (fn x => map (fn y => f x y) ys) xs); 
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225 

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fun extract_first p = 
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let fun extract xs (y::ys) = if p y then (Some y,xs@ys) 
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else extract (y::xs) ys 
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 extract xs [] = (None,xs) 
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in extract [] end; 
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231 

6056  232 
fun print_ineqs ineqs = 
9073  233 
if !trace then 
234 
writeln(cat_lines(""::map (fn Lineq(c,t,l,_) => 

235 
string_of_int c ^ 

236 
(case t of Eq => " = "  Lt=> " < "  Le => " <= ") ^ 

237 
commas(map string_of_int l)) ineqs)) 

238 
else (); 

6056  239 

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fun elim ineqs = 
9073  241 
let val dummy = print_ineqs ineqs; 
6056  242 
val (triv,nontriv) = partition is_trivial ineqs in 
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if not(null triv) 
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then case find_first is_answer triv of 
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None => elim nontriv  some => some 
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else 
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if null nontriv then None else 
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let val (eqs,noneqs) = partition (fn (Lineq(_,ty,_,_)) => ty=Eq) nontriv in 
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if not(null eqs) then 
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let val clist = foldl (fn (cs,Lineq(_,_,l,_)) => l union cs) ([],eqs) 
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val sclist = sort (fn (x,y) => int_ord(abs(x),abs(y))) 
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(filter (fn i => i<>0) clist) 
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val c = hd sclist 
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val (Some(eq as Lineq(_,_,ceq,_)),othereqs) = 
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extract_first (fn Lineq(_,_,l,_) => c mem l) eqs 
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val v = find_index (fn k => k=c) ceq 
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val (ioth,roth) = partition (fn (Lineq(_,_,l,_)) => el v l = 0) 
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(othereqs @ noneqs) 
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val others = map (elim_var v eq) roth @ ioth 
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in elim others end 
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else 
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let val lists = map (fn (Lineq(_,_,l,_)) => l) noneqs 
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val numlist = 0 upto (length(hd lists)  1) 
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val coeffs = map (fn i => map (el i) lists) numlist 
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val blows = map calc_blowup coeffs 
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val iblows = blows ~~ numlist 
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val nziblows = filter (fn (i,_) => i<>0) iblows 
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in if null nziblows then None else 
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let val (c,v) = hd(sort (fn (x,y) => int_ord(fst(x),fst(y))) nziblows) 
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val (no,yes) = partition (fn (Lineq(_,_,l,_)) => el v l = 0) ineqs 
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val (pos,neg) = partition(fn (Lineq(_,_,l,_)) => el v l > 0) yes 
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in elim (no @ allpairs (elim_var v) pos neg) end 
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end 
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274 
end 
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275 
end; 
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276 

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(*  *) 
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(* Translate back a proof. *) 
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279 
(*  *) 
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280 

9073  281 
fun trace_thm msg th = 
282 
if !trace then (writeln msg; prth th) else th; 

283 

284 
fun trace_msg msg = 

285 
if !trace then writeln msg else (); 

286 

6056  287 
(* FIXME OPTIMIZE!!!! 
288 
Addition/Multiplication need i*t representation rather than t+t+... 

10691  289 
Get rid of Mulitplied(2). For Nat LA_Data.number_of should return Suc^n 
290 
because Numerals are not known early enough. 

6056  291 

292 
Simplification may detect a contradiction 'prematurely' due to type 

293 
information: n+1 <= 0 is simplified to False and does not need to be crossed 

294 
with 0 <= n. 

295 
*) 

296 
local 

297 
exception FalseE of thm 

298 
in 

6074  299 
fun mkthm sg asms just = 
10691  300 
let val {add_mono_thms, mult_mono_thms, inj_thms, lessD, simpset} = Data.get_sg sg; 
9420  301 
val atoms = foldl (fn (ats,(lhs,_,_,rhs,_,_)) => 
6056  302 
map fst lhs union (map fst rhs union ats)) 
9420  303 
([], mapfilter (LA_Data.decomp sg o concl_of) asms) 
6056  304 

10575  305 
fun add2 thm1 thm2 = 
6102  306 
let val conj = thm1 RS (thm2 RS LA_Logic.conjI) 
10575  307 
in get_first (fn th => Some(conj RS th) handle _ => None) add_mono_thms 
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308 
end; 
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309 

10575  310 
fun try_add [] _ = None 
311 
 try_add (thm1::thm1s) thm2 = case add2 thm1 thm2 of 

312 
None => try_add thm1s thm2  some => some; 

313 

314 
fun addthms thm1 thm2 = 

315 
case add2 thm1 thm2 of 

316 
None => (case try_add ([thm1] RL inj_thms) thm2 of 

317 
None => the(try_add ([thm2] RL inj_thms) thm1) 

318 
 Some thm => thm) 

319 
 Some thm => thm; 

320 

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321 
fun multn(n,thm) = 
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322 
let fun mul(i,th) = if i=1 then th else mul(i1, addthms thm th) 
6102  323 
in if n < 0 then mul(~n,thm) RS LA_Logic.sym else mul(n,thm) end; 
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324 

10691  325 
fun multn2(n,thm) = 
326 
let val Some(mth,cv) = 

327 
get_first (fn (th,cv) => Some(thm RS th,cv) handle _ => None) mult_mono_thms 

328 
val ct = cterm_of sg (LA_Data.number_of(n,#T(rep_cterm cv))) 

329 
in instantiate ([],[(cv,ct)]) mth end 

330 

6056  331 
fun simp thm = 
10691  332 
let val thm' = full_simplify simpset thm 
6102  333 
in if LA_Logic.is_False thm' then raise FalseE thm' else thm' end 
6056  334 

9073  335 
fun mk(Asm i) = trace_thm "Asm" (nth_elem(i,asms)) 
336 
 mk(Nat(i)) = (trace_msg "Nat"; 

337 
LA_Logic.mk_nat_thm sg (nth_elem(i,atoms))) 

9420  338 
 mk(LessD(j)) = trace_thm "L" (hd([mk j] RL lessD)) 
9073  339 
 mk(NotLeD(j)) = trace_thm "NLe" (mk j RS LA_Logic.not_leD) 
9420  340 
 mk(NotLeDD(j)) = trace_thm "NLeD" (hd([mk j RS LA_Logic.not_leD] RL lessD)) 
9073  341 
 mk(NotLessD(j)) = trace_thm "NL" (mk j RS LA_Logic.not_lessD) 
342 
 mk(Added(j1,j2)) = simp (trace_thm "+" (addthms (mk j1) (mk j2))) 

10717  343 
 mk(Multiplied(n,j)) = (trace_msg("*"^string_of_int n); trace_thm "*" (multn(n,mk j))) 
344 
 mk(Multiplied2(n,j)) = simp (trace_msg("**"^string_of_int n); trace_thm "**" (multn2(n,mk j))) 

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345 

9073  346 
in trace_msg "mkthm"; 
9420  347 
simplify simpset (mk just) handle FalseE thm => thm end 
6056  348 
end; 
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349 

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350 
fun coeff poly atom = case assoc(poly,atom) of None => 0  Some i => i; 
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351 

10691  352 
fun lcms is = foldl lcm (1,is); 
353 

354 
fun integ(rlhs,r,rel,rrhs,s,d) = 

355 
let val (rn,rd) = rep_rat r and (sn,sd) = rep_rat s 

356 
val m = lcms(map (abs o snd o rep_rat) (r :: s :: map snd rlhs @ map snd rrhs)) 

357 
fun mult(t,r) = let val (i,j) = rep_rat r in (t,i * (m div j)) end 

358 
in (m,(map mult rlhs, rn * (m div rd), rel, map mult rrhs, sn * (m div sd), d)) end 

359 

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360 
fun mklineq atoms = 
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361 
let val n = length atoms in 
10691  362 
fn (item,k) => 
363 
let val (m,(lhs,i,rel,rhs,j,discrete)) = integ item 

364 
val lhsa = map (coeff lhs) atoms 

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365 
and rhsa = map (coeff rhs) atoms 
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366 
val diff = map2 (op ) (rhsa,lhsa) 
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367 
val c = ij 
6056  368 
val just = Asm k 
10691  369 
fun lineq(c,le,cs,j) = Some(Lineq(c,le,cs, if m=1 then j else Multiplied2(m,j))) 
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370 
in case rel of 
10691  371 
"<=" => lineq(c,Le,diff,just) 
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372 
 "~<=" => if discrete 
10691  373 
then lineq(1c,Le,map (op ~) diff,NotLeDD(just)) 
374 
else lineq(~c,Lt,map (op ~) diff,NotLeD(just)) 

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375 
 "<" => if discrete 
10691  376 
then lineq(c+1,Le,diff,LessD(just)) 
377 
else lineq(c,Lt,diff,just) 

378 
 "~<" => lineq(~c,Le,map (op~) diff,NotLessD(just)) 

379 
 "=" => lineq(c,Eq,diff,just) 

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380 
 "~=" => None 
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381 
 _ => sys_error("mklineq" ^ rel) 
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382 
end 
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383 
end; 
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384 

6056  385 
fun mknat pTs ixs (atom,i) = 
6128  386 
if LA_Logic.is_nat(pTs,atom) 
6056  387 
then let val l = map (fn j => if j=i then 1 else 0) ixs 
388 
in Some(Lineq(0,Le,l,Nat(i))) end 

389 
else None 

390 

391 
fun abstract pTs items = 

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392 
let val atoms = foldl (fn (ats,((lhs,_,_,rhs,_,_),_)) => 
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393 
(map fst lhs) union ((map fst rhs) union ats)) 
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394 
([],items) 
6056  395 
val ixs = 0 upto (length(atoms)1) 
396 
val iatoms = atoms ~~ ixs 

397 
in mapfilter (mklineq atoms) items @ mapfilter (mknat pTs ixs) iatoms end; 

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398 

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399 
(* Ordinary refutation *) 
6074  400 
fun refute1(pTs,items) = 
401 
(case elim (abstract pTs items) of 

402 
None => [] 

403 
 Some(Lineq(_,_,_,j)) => [j]); 

404 

405 
fun refute1_tac(i,just) = 

406 
fn state => 

407 
let val sg = #sign(rep_thm state) 

6102  408 
in resolve_tac [LA_Logic.notI,LA_Logic.ccontr] i THEN 
6074  409 
METAHYPS (fn asms => rtac (mkthm sg asms just) 1) i 
410 
end 

411 
state; 

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412 

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413 
(* Double refutation caused by equality in conclusion *) 
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414 
fun refute2(pTs,items, (rhs,i,_,lhs,j,d), nHs) = 
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415 
(case elim (abstract pTs (items@[((rhs,i,"<",lhs,j,d),nHs)])) of 
6074  416 
None => [] 
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417 
 Some(Lineq(_,_,_,j1)) => 
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418 
(case elim (abstract pTs (items@[((lhs,j,"<",rhs,i,d),nHs)])) of 
6074  419 
None => [] 
420 
 Some(Lineq(_,_,_,j2)) => [j1,j2])); 

421 

422 
fun refute2_tac(i,just1,just2) = 

423 
fn state => 

424 
let val sg = #sign(rep_thm state) 

6102  425 
in rtac LA_Logic.ccontr i THEN rotate_tac ~1 i THEN etac LA_Logic.neqE i THEN 
6074  426 
METAHYPS (fn asms => rtac (mkthm sg asms just1) 1) i THEN 
427 
METAHYPS (fn asms => rtac (mkthm sg asms just2) 1) i 

428 
end 

429 
state; 

430 

9420  431 
fun prove sg (pTs,Hs,concl) = 
6074  432 
let val nHs = length Hs 
433 
val ixHs = Hs ~~ (0 upto (nHs1)) 

9420  434 
val Hitems = mapfilter (fn (h,i) => case LA_Data.decomp sg h of 
6074  435 
None => None  Some(it) => Some(it,i)) ixHs 
9420  436 
in case LA_Data.decomp sg concl of 
6074  437 
None => if null Hitems then [] else refute1(pTs,Hitems) 
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438 
 Some(citem as (r,i,rel,l,j,d)) => 
6074  439 
if rel = "=" 
440 
then refute2(pTs,Hitems,citem,nHs) 

441 
else let val neg::rel0 = explode rel 

442 
val nrel = if neg = "~" then implode rel0 else "~"^rel 

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443 
in refute1(pTs, Hitems@[((r,i,nrel,l,j,d),nHs)]) end 
6074  444 
end; 
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445 

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446 
(* 
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447 
Fast but very incomplete decider. Only premises and conclusions 
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448 
that are already (negated) (in)equations are taken into account. 
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449 
*) 
9420  450 
fun lin_arith_tac i st = SUBGOAL (fn (A,n) => 
6056  451 
let val pTs = rev(map snd (Logic.strip_params A)) 
452 
val Hs = Logic.strip_assums_hyp A 

6074  453 
val concl = Logic.strip_assums_concl A 
9420  454 
in case prove (Thm.sign_of_thm st) (pTs,Hs,concl) of 
6074  455 
[j] => refute1_tac(n,j) 
456 
 [j1,j2] => refute2_tac(n,j1,j2) 

457 
 _ => no_tac 

9420  458 
end) i st; 
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459 

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460 
fun cut_lin_arith_tac thms i = cut_facts_tac thms i THEN lin_arith_tac i; 
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461 

6079  462 
fun prover1(just,sg,thms,concl,pos) = 
6102  463 
let val nconcl = LA_Logic.neg_prop concl 
6074  464 
val cnconcl = cterm_of sg nconcl 
465 
val Fthm = mkthm sg (thms @ [assume cnconcl]) just 

6102  466 
val contr = if pos then LA_Logic.ccontr else LA_Logic.notI 
467 
in Some(LA_Logic.mk_Eq ((implies_intr cnconcl Fthm) COMP contr)) end 

6074  468 
handle _ => None; 
469 

470 
(* handle thm with equality conclusion *) 

471 
fun prover2(just1,just2,sg,thms,concl) = 

6102  472 
let val nconcl = LA_Logic.neg_prop concl (* m ~= n *) 
6074  473 
val cnconcl = cterm_of sg nconcl 
474 
val neqthm = assume cnconcl 

6102  475 
val casethm = neqthm COMP LA_Logic.neqE (* [m<n ==> R; n<m ==> R] ==> R *) 
6074  476 
val [lessimp1,lessimp2] = prems_of casethm 
477 
val less1 = fst(Logic.dest_implies lessimp1) (* m<n *) 

478 
and less2 = fst(Logic.dest_implies lessimp2) (* n<m *) 

479 
val cless1 = cterm_of sg less1 and cless2 = cterm_of sg less2 

480 
val thm1 = mkthm sg (thms @ [assume cless1]) just1 

481 
and thm2 = mkthm sg (thms @ [assume cless2]) just2 

482 
val dthm1 = implies_intr cless1 thm1 and dthm2 = implies_intr cless2 thm2 

483 
val thm = dthm2 COMP (dthm1 COMP casethm) 

6102  484 
in Some(LA_Logic.mk_Eq ((implies_intr cnconcl thm) COMP LA_Logic.ccontr)) end 
6074  485 
handle _ => None; 
486 

6079  487 
(* PRE: concl is not negated! *) 
6074  488 
fun lin_arith_prover sg thms concl = 
489 
let val Hs = map (#prop o rep_thm) thms 

6102  490 
val Tconcl = LA_Logic.mk_Trueprop concl 
9420  491 
in case prove sg ([],Hs,Tconcl) of 
6079  492 
[j] => prover1(j,sg,thms,Tconcl,true) 
6074  493 
 [j1,j2] => prover2(j1,j2,sg,thms,Tconcl) 
6102  494 
 _ => let val nTconcl = LA_Logic.neg_prop Tconcl 
9420  495 
in case prove sg ([],Hs,nTconcl) of 
6079  496 
[j] => prover1(j,sg,thms,nTconcl,false) 
497 
(* [_,_] impossible because of negation *) 

498 
 _ => None 

499 
end 

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500 
end; 
6074  501 

502 
end; 