Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  f : A → A
  tl' : list A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  IHl : ∀ l' : list A,
          OnOne2 P (map f l) l'
          → ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) l l''
  o : OnOne2 P (map f l) tl'
  x : list A
  o0 : OnOne2 (λ x y : A, P (f x) (f y)) l x
  ============================
   (OnOne2 (λ x y : A, P (f x) (f y)) l ?tl')


DEBUG:Going to execute:
<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
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := OnOne2 (λ x y : A, P (f x) (f y)) l ?tl'
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.