diff --git a/src/bisonpre b/src/bisonpre
index c60e3575d..48f1761ff 100755
--- a/src/bisonpre
+++ b/src/bisonpre
@@ -17,20 +17,28 @@ def process():
 
     bison_version_check()
     supports_report = Bison_Version >= 2.3
+    supports_counter_examples = Bison_Version >= 3.8
 
     clean_input(Args.input, tmp_prefix() + ".y")
 
     # Run bison
     command = (
-        Args.yacc + (" -t" if Args.debug else "") +
-        (" -d" if Args.definitions else "") +
-        (" -k" if Args.token_table else "") + (" -v" if Args.verbose else "") +
-        (" --report=itemset --report=lookahead" if
-         (Args.verbose and supports_report) else "")
+        Args.yacc  #
+        + (" -t" if Args.debug else "")  #
+        + (" -d" if Args.definitions else "")  #
+        + (" -k" if Args.token_table else "")  #
+        + (" -v" if Args.verbose else "")  #
+        + (" --report=itemset --report=lookahead" if
+           (Args.verbose and supports_report) else "")
+        # Useful but slow:
+        # (" -Wcounterexamples" if
+        # (Args.verbose and supports_counter_examples) else "")
+        #
         # -p required for GLR parsers; they write to -p basename, not -o
         + ((" -p " + Args.name_prefix) if Args.name_prefix else "") + " -b " +
-        tmp_prefix() + " -o " + tmp_prefix() + ".c" + " " + tmp_prefix() +
-        ".y")
+        tmp_prefix()  #
+        + " -o " + tmp_prefix() + ".c"  #
+        + " " + tmp_prefix() + ".y")
 
     print("  " + command)
     status = subprocess.call(command, shell=True)