
tcfail017.hs:10:10:
    Could not deduce (C [a])
      arising from the superclasses of an instance declaration
    from the context (B a)
      bound by the instance declaration at tcfail017.hs:10:10-23
    Possible fix:
      add (C [a]) to the context of the instance declaration
      or add an instance declaration for (C [a])
    In the instance declaration for `B [a]'
