3 Could not deduce `C [a]'
4 (arising from an instance declaration at tcfail019.hs:19)
6 Probable cause: missing `C [a]' in instance declaration context
7 or missing instance declaration for `C [a]'
8 When checking the superclasses of an instance declaration
11 Could not deduce `B [a]'
12 (arising from an instance declaration at tcfail019.hs:19)
14 Probable cause: missing `B [a]' in instance declaration context
15 or missing instance declaration for `B [a]'
16 When checking the superclasses of an instance declaration
19 Compilation had errors