Goal:
  
  A : Type
  t : A
  P : A → A → Type
  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'
  ============================
   (t = t)


DEBUG:Going to execute:
fun tauto_flags X1 => <coq-core.plugins.tauto::is_unit_or_eq@0>