Goal:
    A : Type  B : Type  C : Type  R : nat → A → C → Type  g : A → B → C
  ============================
   (∀ k : nat,
      All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k [] []
      → All2i R k [] [])


DEBUG:Going to execute:
trivial; hnf