size :: Num n => Para.Alg (F.Tree k v) n
size = Para.Psi $ \f ->
  case f of
    F.Leaf                     -> 0
    F.Branch _ _ (_, l) (_, r) -> 1 + l + r
size :: Num n => Para.Alg (F.Tree k v) n
size = Para.Psi $ \f ->
  case f of
    F.Leaf                     -> 0
    F.Branch _ _ (_, l) (_, r) -> 1 + l + r
size = Para.Psi $ \f ->
  case f of
    F.Leaf                     -> 0
    F.Branch _ _ (_, l) (_, r) -> 1 + l + r