tcfail019.hs:19:
    Could not deduce `C [a]'
	(arising from an instance declaration at tcfail019.hs:19)
	from the context ()
    Probable cause: missing `C [a]' in instance declaration context
		    or missing instance declaration for `C [a]'
    When checking the superclasses of an instance declaration
 
tcfail019.hs:19:
    Could not deduce `B [a]'
	(arising from an instance declaration at tcfail019.hs:19)
	from the context ()
    Probable cause: missing `B [a]' in instance declaration context
		    or missing instance declaration for `B [a]'
    When checking the superclasses of an instance declaration


Compilation had errors