Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  a0 : A
  l' : list A
  r : A
  r' : A
  IHl : ∀ l' : list A, All2 P (l ++ [r]) (l' ++ [r']) → All2 P l l' × P r r'
  X : All2 P (a :: l ++ [r]) ((a0 :: l') ++ [r'])
  X0 : P a a0
  a1 : All2 P l l'
  b : P r r'
  ============================
   (P r r')


DEBUG:Going to execute:
<Coq.Init.Tauto.flatten_contravariant_conj> flags X1 X2 id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := A
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.