T3208b.hs:15:15: error:
    • Could not deduce: OTerm o0 ~ STerm a
        arising from a use of ‘apply’
      from the context: (OTerm a ~ STerm a, OBJECT a, SUBST a)
        bound by the type signature for:
                   fce' :: forall a c.
                           (OTerm a ~ STerm a, OBJECT a, SUBST a) =>
                           a -> c
        at T3208b.hs:14:1-56
      The type variable ‘o0’ is ambiguous
    • In the first argument of ‘fce’, namely ‘(apply f)’
      In the expression: fce (apply f)
      In an equation for ‘fce'’: fce' f = fce (apply f)
    • Relevant bindings include
        f :: a (bound at T3208b.hs:15:6)
        fce' :: a -> c (bound at T3208b.hs:15:1)