Goal:
  
  A : Type
  n : nat
  t : A
  P : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  H : nth_error tl n = Some t
  ============================
   (nth_error tl n = Some t × (t = t) + P (i + S n) t t)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro