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:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
Level 0: Backtrace:

P is used in hypothesis X.
Level 0: Backtrace:

P is used in hypothesis X.
Level 0: Backtrace:

P is used in hypothesis X.
Level 0: Backtrace:

P is used in hypothesis X.
Level 0: Backtrace:

P is used in hypothesis X.
Level 1: Backtrace:

P is used in hypothesis X.
Level 0: Backtrace:

P is used in hypothesis X.
Level 3: Backtrace:

Not convertible.
Level 3: Backtrace:

Not convertible.
Level 3: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.