Goal:
  
  A : Type
  P : A → A → Prop
  r : A
  r' : A
  H : Forall2 P [r] [r']
  H3 : P r r'
  H5 : Forall2 P [] []
  ============================
   (P r r')


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 := P r r'
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.