T9951b.hs:7:1: warning: [GHC-62161] [-Wincomplete-patterns (in -Wextra)]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns of type ‘String’ not matched:
            []
            (p:_) where p is not one of {'a'}
            ['a']
            ('a':p:_) where p is not one of {'b'}
            ...