- "none - no warnings",
- "error - warnings are errors",
- "yacc - incompatibilities with POSIX YACC",
- "all - all of the above",
+ "none - no warnings",
+ "midrule-values - unset or unused midrule values",
+ "yacc - incompatibilities with POSIX YACC",
+ "all - all of the above",
+ "error - warnings are errors",