T9951b.hs:7:1: warning: [-Wincomplete-patterns (in -Wextra)]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns not matched:
            []
            [p] where p is not one of {'a'}
            (p:_:_) where p is not one of {'a'}
            ['a']
            ...