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) + P t t)%type


DEBUG:Going to execute:
match reverse goal with
           | id:(?X1 → ?X2) → ?X3
             |- _ =>
                 Ltac.cut X3;
                  [ intro; clear id; t_tauto_intuit
                  | Ltac.cut (X1 → X2);
                     [ exact
                     id
                     | generalize (λ y : X2, id (λ x : X1, y)); intro; clear
                        id; (solve [ t_tauto_intuit ]) ] ]
           | id:¬ ?X1 → ?X3
             |- _ =>
                 Ltac.cut X3;
                  [ intro; clear id; t_tauto_intuit
                  | Ltac.cut (¬ X1);
                     [ exact
                     id
                     | clear id; intro; (solve [ t_tauto_intuit ]) ] ]
           | |- ?X1 =>
                 <Coq.Init.Tauto.is_disj> flags X1; (solve
                  [ Ltac.left; t_tauto_intuit | Ltac.right; t_tauto_intuit ])
           end ||
             match goal with
             | |- _ → _ => intro; t_tauto_intuit
             | |- _ => t_reduce; t_solver
             end || t_solver
Level 3: Backtrace:

No such assumption.
Level 2: Backtrace:

No such assumption.
Level 1: Backtrace:

No such assumption.
Level 1: evaluation raised the exception
Backtrace:

No such assumption.

Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.