Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  h : OnOne2 P tl tl'
  x : A
  y : A
  u : list A
  v : list A
  a : P x y
  H : tl = u ++ x :: v
  H0 : tl' = u ++ y :: v
  ============================
   (P x y)


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.