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'
  ============================
   (Some y = Some y ∧ P t y)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
Level 0: Backtrace:

P is used in conclusion.
Level 0: Backtrace:

P is used in conclusion.
Level 0: Backtrace:

P is used in conclusion.
Level 0: Backtrace:

P is used in conclusion.
Level 0: Backtrace:

P is used in conclusion.
Level 1: Backtrace:

P is used in conclusion.
Level 0: Backtrace:

P is used in conclusion.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 1: Backtrace:

Not convertible.