Maybe.bind
: ∀(A: *)
  ∀(B: *)
  ∀(a: (Maybe A))
  ∀(b: ∀(x: A) (Maybe B))
  (Maybe B)
= λA λB λa λb
  use P = λx ∀(b: ∀(x: A) (Maybe B)) (Maybe B)
  use some = λa.value λb (b a.value)
  use none = λb (Maybe.none B)
  (~a P some none b)