Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : nat → A → B → Type
  l : list C
  l' : list D
  f : nat → C → A
  g : nat → D → B
  n : nat
  ============================
   (All2i (λ (n : nat) (x : C) (y : D), R n (f n x) (g n y)) n l l'
    → All2i R n (mapi_rec f l n) (mapi_rec g l' n))


DEBUG:Going to execute:
induction 1; constructor