SimpleFail5a.hs:31:11:
    Couldn't match type `a' with `Int'
      `a' is a rigid type variable bound by
          the type signature for bar3wrong :: S3 a -> a
          at SimpleFail5a.hs:31:1
    Expected type: S3 a
      Actual type: S3 Int
    In the pattern: D3Int
    In an equation for `bar3wrong': bar3wrong D3Int = 1