T2714.hs:8:5:
    Couldn't match type ‘a’ with ‘f0 b’
      ‘a’ is a rigid type variable bound by
          the type signature for: f :: ((a -> b) -> b) -> forall c. c -> a
          at T2714.hs:7:6
    Expected type: ((a -> b) -> b) -> c -> a
      Actual type: ((a -> b) -> b) -> f0 (a -> b) -> f0 b
    In the expression: ffmap
    In an equation for ‘f’: f = ffmap
    Relevant bindings include
      f :: ((a -> b) -> b) -> forall c. c -> a (bound at T2714.hs:8:1)

T2714.hs:8:5:
    Couldn't match type ‘c’ with ‘f0 (a -> b)’
      ‘c’ is a rigid type variable bound by
          the type signature for: f :: ((a -> b) -> b) -> c -> a
          at T2714.hs:8:1
    Expected type: ((a -> b) -> b) -> c -> a
      Actual type: ((a -> b) -> b) -> f0 (a -> b) -> f0 b
    In the expression: ffmap
    In an equation for ‘f’: f = ffmap
    Relevant bindings include
      f :: ((a -> b) -> b) -> forall c. c -> a (bound at T2714.hs:8:1)