Goal:
  
  A : Type
  P : nat → A → A → Type
  Q : A → Type
  hd : A
  tl : list A
  i : nat
  tl' : list A
  X : OnOne2i P (S i) tl tl'
  X0 : Q hd
  X1 : All Q tl
  X2 : OnOne2i (λ (i : nat) (x y : A), P i x y × Q x) (S i) tl tl'
  ============================
   (OnOne2i (λ (i : nat) (x y : A), P i x y × Q x) (S i) tl tl')


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 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := nat
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 1: Backtrace:

Tactic failure.