Goal:
  
  A : Type
  B : Type
  C : Type
  k : nat
  R : nat → A → C → Type
  g : A → B → C
  l : list A
  l' : list B
  ============================
   (All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k l l'
    → All2i R k l (map2 g l l'))


DEBUG:Going to execute:
elim : l l' k  => [|x xs ih] [|y ys] //= k z