tcfail071.hs:5:
    Mismatched contexts
    When matching the contexts of the signatures for `g' and `f'
	(the signature contexts in a mutually recursive group should all be identical)

tcfail071.hs:10:
    Cannot unify the type-signature variable `c'
	with the type `a -> b -> b'
	Expected type: a -> b -> b
	Inferred type: c
    In the first argument of `f', namely `c'
    In the first argument of `foldr', namely `(f c)'