Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  H : OnOne2 P tl tl'
  HPQ : ∀ x y : A, P x y → ∑ z : A, Q x z × Q y z
  r : list A
  o : OnOne2 Q tl r
  o0 : OnOne2 Q tl' r
  ============================
   (OnOne2 Q (hd :: tl) (hd :: r))


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := OnOne2 P 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.