Warning categories include:
'conflicts-sr' S/R conflicts (enabled by default)
'conflicts-rr' R/R conflicts (enabled by default)
'deprecated' obsolete constructs
'empty-rule' empty rules without
%e
mpty
'midrule-values' unset or unused midrule values
'precedence' useless precedence and associativity
'yacc' incompatibilities with POSIX Yacc
'other' all other warnings (enabled by default)
'all' all the warnings except 'yacc'
'no-CATEGORY' turn off warnings in CATEGORY
'none' turn off all the warnings
'error[=CATEGORY]' treat warnings as errors