tcfail170.hs:7:10: error:
    Illegal instance declaration for ‘C [p] [q]’
      The coverage condition fails in class ‘C’
        for functional dependency: ‘a -> b’
      Reason: lhs type ‘[p]’ does not determine rhs type ‘[q]’
      Un-determined variable: q
    In the instance declaration for ‘C [p] [q]’