added same_fstI as safe intro rule

--- a/src/HOL/Wellfounded_Relations.ML	Thu May 31 16:50:15 2001 +0200
+++ b/src/HOL/Wellfounded_Relations.ML	Thu May 31 16:50:16 2001 +0200
@@ -208,6 +208,7 @@
 Goalw[same_fst_def] "[| P x; (y',y) : R x |] ==> ((x,y'),(x,y)) : same_fst P R";
 by (Asm_simp_tac 1);
 qed "same_fstI";
+AddSIs[same_fstI];
 
 val prems = goalw thy [same_fst_def]
   "(!!x. P x ==> wf(R x)) ==> wf(same_fst P R)";