Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  R : A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  X0 : Q hd
  X1 : All Q tl
  X2 : R hd
  X3 : All R tl'
  X5 : OnOne2 (λ x y : A, P x y × Q x × R y) tl tl'
  ============================
   (OnOne2 (λ x y : A, P x y × Q x × R y) tl tl')


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := OnOne2 (λ x y : A, P x y × Q x × R y) tl tl'
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.