tcfail037.hs:7:11:
    Ambiguous occurrence `+'
    It could refer to either `ShouldFail.+',
                             defined at tcfail037.hs:10:5
                          or `Prelude.+',
                             imported from `Prelude' at tcfail037.hs:3:8-17
                             (and originally defined in `GHC.Num')