Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  l : list A
  l' : list B
  t' : B
  Hall : All2 P l l'
  ============================
   (∀ n : nat,
      nth_error l' n = Some t' → ∑ t : A, nth_error l n = Some t × P t t')


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