Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  l : list A
  l' : list B
  t : A
  t' : B
  Hall : Forall2 P l l'
  ============================
   (∀ n : nat, nth_error l n = Some t → nth_error l' n = Some t' → P t t')


DEBUG:Going to execute:
induction Hall; destruct n; simpl