YAP would break when gc was called from portray:

always save arguments before calling plwrite with possible portray
 only do it for portray because plwrite may be called from unsafe environments
 make Slot machinery mainstream.


git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@637 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

C/heapgc.c

@@ -3020,6 +3020,9 @@ do_gc(Int predarity, CELL *current_env, yamop *nextop)
     return(0);
   }
   gc_calls++;
+  if (gc_calls == 19) {
+    fprintf(stderr,"here I go\n");
+  }
   if (gc_trace) {
     YP_fprintf(YP_stderr, "[gc]\n");
   } else if (gc_verbose) {