Index: class.c
==================================================================
--- class.c
+++ class.c
@@ -1155,15 +1155,16 @@
           cl->codes[ptr++]=0;
           depth++;
           break;
         case OP_ELSE:
           if(!depth) ParseError("Premature end of subpattern\n");
-          cl->codes[nest[depth-1]+1]=ptr;
+          cl->codes[nest[depth-1]+1]=ptr+1;
           cl->codes[ptr++]=OP_ELSE;
           cl->codes[ptr++]=cl->codes[nest[depth-1]];
           cl->codes[nest[depth-1]]=OP_IF;
           cl->codes[ptr++]=0;
+          nest[depth-1]=ptr-2;
           break;
         case OP_THEN:
           if(!depth) ParseError("Premature end of subpattern\n");
           cl->codes[nest[--depth]+1]=ptr;
           cl->codes[ptr++]=OP_THEN;

Index: exec.c
==================================================================
--- exec.c
+++ exec.c
@@ -1864,11 +1864,13 @@
       if(main_options['t']) {
         printf("ptr=%d cpi=%d x=%d y=%d dir=%d obj=%lu ",ptr,cpi,x,y,d,(long)n);
         printf("[ptr=%d x=%d y=%d dir=%d]\n",cp[cpi].ptr,cp[cpi].x,cp[cpi].y,cp[cpi].dir);
       }
       break;
-    default: Throw("Unimplemented opcode in pattern");
+    default:
+      fprintf(stderr,"Unrecognized opcode 0x%04X at 0x%04X in pattern\n",code[ptr-1],ptr-1);
+      Throw("Internal error: Unimplemented opcode in pattern");
   }
   goto again;
   fail:
   if(!all) {
     if(vstackptr<cp->depth) Throw("Stack underflow in pattern matching");