3 Could not deduce `C [a]' from the context ()
5 Add `C [a]' to the instance declaration context
6 Or add an instance declaration for `C [a]'
7 arising from an instance declaration at tcfail019.hs:18
10 Could not deduce `B [a]' from the context ()
12 Add `B [a]' to the instance declaration context
13 Or add an instance declaration for `B [a]'
14 arising from an instance declaration at tcfail019.hs:18