# HG changeset patch
# User paulson
# Date 975489758 -3600
# Node ID 1d2f15504d3830c379a0007f95991e21e46734ad
# Parent  8f34ecae14468fc1d5d6ecebb061f9c98c172a98
simproc for cancelling common factors around = < <= div /

diff -r 8f34ecae1446 -r 1d2f15504d38 src/Provers/Arith/cancel_numeral_factor.ML
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Provers/Arith/cancel_numeral_factor.ML	Wed Nov 29 10:22:38 2000 +0100
@@ -0,0 +1,87 @@
+(*  Title:      Provers/Arith/cancel_numeral_factor.ML
+    ID:         $Id$
+    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
+    Copyright   2000  University of Cambridge
+
+Cancel common coefficients in balanced expressions:
+
+     u*#m ~~ u'*#m'  ==  #n*u ~~ #n'*u'
+
+where ~~ is an appropriate balancing operation (e.g. =, <=, <, div, /)
+and d = gcd(m,m') and n=m/d and n'=m'/d.
+
+It works by (a) massaging both sides to bring gcd(m,m') to the front:
+
+     u*#m ~~ u'*#m'  ==  #d*(#n*u) ~~ #d*(#n'*u')
+
+(b) then using the rule "cancel" to reach #n*u ~~ #n'*u'.
+*)
+
+signature CANCEL_NUMERAL_FACTOR_DATA =
+sig
+  (*abstract syntax*)
+  val mk_bal: term * term -> term
+  val dest_bal: term -> term * term
+  val mk_coeff: int * term -> term
+  val dest_coeff: term -> int * term
+  (*rules*)
+  val cancel: thm
+  val neg_exchanges: bool  (*true if a negative coeff swaps the two operands,
+                             as with < and <= but not = and div*)
+  (*proof tools*)
+  val prove_conv: tactic list -> Sign.sg -> 
+                  thm list -> term * term -> thm option
+  val trans_tac: thm option -> tactic (*applies the initial lemma*)
+  val norm_tac: tactic                (*proves the initial lemma*)
+  val numeral_simp_tac: tactic        (*proves the final theorem*)
+  val simplify_meta_eq: thm -> thm    (*simplifies the final theorem*)
+end;
+
+
+functor CancelNumeralFactorFun(Data: CANCEL_NUMERAL_FACTOR_DATA):
+  sig
+  val proc: Sign.sg -> thm list -> term -> thm option
+  end 
+=
+struct
+
+
+(* greatest common divisor *)
+fun gcd (0, n) = abs n
+  | gcd (m, n) = gcd (n mod m, m);
+
+(*the simplification procedure*)
+fun proc sg hyps t =
+  let (*first freeze any Vars in the term to prevent flex-flex problems*)
+      val rand_s = gensym"_"
+      fun mk_inst (var as Var((a,i),T))  = 
+	    (var,  Free((a ^ rand_s ^ string_of_int i), T))
+      val t' = subst_atomic (map mk_inst (term_vars t)) t
+      val (t1,t2) = Data.dest_bal t' 
+      val (m1, t1') = Data.dest_coeff t1
+      and (m2, t2') = Data.dest_coeff t2
+      val d = (*if both are negative, also divide through by ~1*)
+          if m1<0 andalso m2<0 then ~ (gcd(m1,m2)) else gcd(m1,m2)
+      val _ = if d=1 then   (*trivial, so do nothing*)
+		      raise TERM("cancel_numeral_factor", []) 
+              else ()
+      fun newshape (i,t) = Data.mk_coeff(d, Data.mk_coeff(i,t))
+      val n1 = m1 div d and n2 = m2 div d
+      val rhs = if d<0 andalso Data.neg_exchanges
+                then Data.mk_bal (Data.mk_coeff(n2,t2'), Data.mk_coeff(n1,t1'))
+                else Data.mk_bal (Data.mk_coeff(n1,t1'), Data.mk_coeff(n2,t2'))
+      val reshape =  (*Move d to the front and put the rest into standard form
+		       i * #m * j == #d * (#n * (j * k)) *)
+	    Data.prove_conv [Data.norm_tac] sg hyps 
+	      (t',   Data.mk_bal (newshape(n1,t1'), newshape(n2,t2')))
+  in
+      apsome Data.simplify_meta_eq
+       (Data.prove_conv 
+	       [Data.trans_tac reshape, rtac Data.cancel 1,
+		Data.numeral_simp_tac] sg hyps (t', rhs))
+  end
+  handle TERM _ => None
+       | TYPE _ => None;   (*Typically (if thy doesn't include Numeral)
+			     Undeclared type constructor "Numeral.bin"*)
+
+end;