trim_trail for default machine

be more aggressive about try-retry-trust chains.
   - handle cases where block starts with a wait
   - don't use _killed instructions, just let the thing rot by itself.


git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@1027 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

C/tracer.c

@@ -127,6 +127,10 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   }
 #endif
   vsc_count++;
+  if (vsc_count < 5319900)
+    return;
+  if (vsc_count == 5319949)
+    vsc_xstop = 1;
 #ifdef COMMENTED
   //  if (vsc_count == 218280)
   //    vsc_xstop = 1;