arith_tac now copes with propositional reasoning as well.

--- a/src/HOL/simpdata.ML	Wed Mar 07 18:35:27 2001 +0100
+++ b/src/HOL/simpdata.ML	Fri Mar 09 19:05:48 2001 +0100
@@ -487,7 +487,8 @@
         REPEAT(eresolve_tac [conjE, exE] 1 ORELSE
                filter_prems_tac test 1 ORELSE
                etac disjE 1) THEN
-        ref_tac 1;
+        ((etac notE 1 THEN eq_assume_tac 1) ORELSE
+         ref_tac 1);
   in EVERY'[TRY o filter_prems_tac test,
             DETERM o REPEAT o etac rev_mp, prep_tac, rtac ccontr, prem_nnf_tac,
             SELECT_GOAL (DEPTH_SOLVE refute_prems_tac)]