Goal:
  
  A : Type
  n : nat
  t' : A
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : ∀ n : nat,
          nth_error tl' n = Some t'
          → ∑ t : A, nth_error tl n = Some t × (t = t') + P t t'
  ============================
   (nth_error (hd :: tl') n = Some t'
    → ∑ t : A, nth_error (hd :: tl) n = Some t × (t = t') + P t t')


DEBUG:Going to execute:
destruct n; simpl