T8044.hs:16:13:
    Couldn't match type ‘Frob a’ with ‘Char’
    Expected type: X (Frob a)
      Actual type: X Char
    In the expression: XChar
    In an equation for ‘frob’: frob _ = XChar
    Relevant bindings include
      frob :: X a -> X (Frob a) (bound at T8044.hs:15:1)