lib: Produce partial counterexamples, but be faster

lib/grammars_cfg.py

@@ -626,7 +626,9 @@ class CFG:
 
                     if max(nxl, nxr) > full_cmp_cnt or min_size > max_cmp_len:
                         break
-                if left_ce is not None and right_ce is not None:
+
+                # produce counterexample even if it is only partial
+                if left_ce is not None or right_ce is not None:
                     return mkres()
 
         # produce counterexample even if it is only partial