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


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end
Level 0: Backtrace:

No matching clauses for match.