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


DEBUG:Going to execute:
fun flags =>
             match reverse goal with
             | |- ?X1 =>
                   <Coq.Init.Tauto.is_unit_or_eq> flags X1; constructor 1
             | H:?X1 |- _ => <Coq.Init.Tauto.is_empty> flags X1; elim H
             | _ => Ltac.assumption
             end