pmc001.hs:14:1: warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns not matched:
            MkT3 (MkT2 _)
            MkT3 MkT1
            (MkT2 _) MkT3
            MkT1 MkT3

pmc001.hs:19:1: warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘g’:
        Patterns not matched:
            MkT3 (MkT2 _)
            MkT3 MkT1
            (MkT2 _) MkT3
            MkT1 MkT3