Goal:
  
  A : Type
  t' : A
  P : nat → A → A → Type
  i : nat
  tl : list A
  tl' : list A
  X : OnOne2i P (S i) tl tl'
  IHX : ∀ n : nat,
          nth_error tl' n = Some t'
          → ∑ t : A,
              nth_error tl n = Some t × (t = t') + P (S i + n)%nat t t'
  ============================
   (t' = t')


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