T5853.hs:15:46: error:
    • Could not deduce: Subst (Subst fa a) b ~ Subst fa b
        arising from a use of ‘<$>’
      from the context: (F fa,
                         Elem fa ~ Elem fa,
                         Elem (Subst fa b) ~ b,
                         Subst fa b ~ Subst fa b,
                         Subst (Subst fa b) (Elem fa) ~ fa,
                         F (Subst fa a),
                         Elem (Subst fa a) ~ a,
                         Elem fa ~ Elem fa,
                         Subst (Subst fa a) (Elem fa) ~ fa,
                         Subst fa a ~ Subst fa a)
        bound by the RULE "map/map" at T5853.hs:15:2-57
      NB: ‘Subst’ is a type function, and may not be injective
    • In the expression: (f . g) <$> xs
      When checking the transformation rule "map/map"
    • Relevant bindings include
        f :: Elem fa -> b (bound at T5853.hs:15:19)
        g :: a -> Elem fa (bound at T5853.hs:15:21)
        xs :: Subst fa a (bound at T5853.hs:15:23)