Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : P x y
  Hall : All2 P l l'
  IHHall : ∀ n : nat, nth_error l n = None → nth_error l' n = None
  ============================
   (Some x = None → Some y = None)


DEBUG:Going to execute:
congruence