completesig04.hs:10:1: warning: [-Wincomplete-patterns (in -Wextra)]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns of type  ‘T’ not matched:
            B
            C

completesig04.hs:12:1: warning: [-Wincomplete-patterns (in -Wextra)]
    Pattern match(es) are non-exhaustive
    In an equation for ‘g’:
        Patterns of type  ‘Maybe a’ not matched: Nothing