Editing GNU Bison (section)

===Counterexample generation===
One delicate issue with LR parser generators is the resolution of conflicts (shift/reduce and reduce/reduce conflicts).  With many LR parser generators, resolving conflicts requires the analysis of the parser automaton, which demands some expertise from the user.

To aid the user in understanding conflicts more intuitively, Bison can instead automatically generate counterexamples. For [[ambiguous grammar]]s, Bison often can even produce counterexamples that show the grammar is ambiguous.

For instance, on a grammar suffering from the infamous [[dangling else]] problem, Bison reports

{{Pre|
doc/if-then-else.y: {{font color|purple|warning}}: shift/reduce conflict on token "else" [-{{font color|purple|Wcounterexamples}}]
  Example: {{font color|orange|"if" expr "then"}} {{font color|blue|"if" expr "then" stmt}} {{font color|red|•}} {{font color|blue|"else" stmt}}
  Shift derivation
    {{font color|orange|if_stmt}}
    {{font color|orange|↳ "if" expr "then"}} {{font color|green|stmt}}
                        {{font color|green|↳}} {{font color|blue|if_stmt}}
                           {{font color|blue|↳ "if" expr "then" stmt}} {{font color|red|•}} {{font color|blue|"else" stmt}}
  Example: {{font color|orange|"if" expr "then"}} {{font color|blue|"if" expr "then" stmt}} {{font color|red|•}} {{font color|orange|"else" stmt}}
  Reduce derivation
    {{font color|orange|if_stmt}}
    {{font color|orange|↳ "if" expr "then"}} {{font color|green|stmt}}                        {{font color|orange|"else" stmt}}
                        {{font color|green|↳}} {{font color|blue|if_stmt}}
                           {{font color|blue|↳ "if" expr "then" stmt}} {{font color|red|•}}
}}