SilentParametersOverlapping.hs:15:9:
    Overlapping instances for C [(t0, t1)] arising from a use of ‘c’
    Matching givens (or their superclasses):
      (C [(a, b)])
        bound by the instance declaration
        at SilentParametersOverlapping.hs:14:37-45
    Matching instances:
      instance C [a] -- Defined at SilentParametersOverlapping.hs:11:10
    (The choice depends on the instantiation of ‘t0, t1’)
    In the expression: c [(undefined, undefined)]
    In an equation for ‘b’: b x = c [(undefined, undefined)]
    In the instance declaration for ‘B [(a, b)]’