T19709c.hs:9:7: error: [-Wincomplete-patterns (in -Wextra), -Werror=incomplete-patterns]
    Pattern match(es) are non-exhaustive
    In a case alternative:
        Patterns of type ‘String’ not matched:
            []
            (p:_) where p is not one of {'h'}
            ['h']
            ('h':p:_) where p is not one of {'e'}
            ...