tcfail072.hs:23:13:
    Could not deduce (Ord q0) arising from a use of ‘g’
    from the context (Ord p, Ord q)
      bound by the type signature for
                 g :: (Ord p, Ord q) => AB p q -> Bool
      at tcfail072.hs:22:6-38
    The type variable ‘q0’ is ambiguous
    Note: there are several potential instances:
      instance Ord a => Ord (Maybe a) -- Defined in ‘GHC.Base’
      instance Ord () -- Defined in ‘GHC.Classes’
      instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
      ...plus 22 others
    In the expression: g A
    In an equation for ‘g’: g (B _ _) = g A