Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  l : list A
  l' : list B
  ============================
   (#|l| = #|l'|
    → All2 R l (map2 g l l') → All2 (λ (x : A) (y : B), R x (g x y)) l l')


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