tcfail019.hs:18:
    Could not deduce `C [a]' from the context ()
    Probable fix:
	Add `C [a]' to the instance declaration context
	Or add an instance declaration for `C [a]'
    arising from an instance declaration at tcfail019.hs:18

tcfail019.hs:18:
    Could not deduce `B [a]' from the context ()
    Probable fix:
	Add `B [a]' to the instance declaration context
	Or add an instance declaration for `B [a]'
    arising from an instance declaration at tcfail019.hs:18