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'
  ============================
   (t' = t')


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