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