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'
  ============================
   (Some t' = Some t' × (t' = t') + P (i + 0) t' t')


DEBUG:Going to execute:
let rec t_tauto_intuit :=
            <Coq.Init.Tauto.simplif> flags;
             <Coq.Init.Tauto.axioms> flags ||
               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
           in
           t_tauto_intuit