Goal:
  
  A : Type
  R : A → A → Prop
  a : A
  l : list A
  IHl : Forall (λ x : A, R x x) l → Forall2 R l l
  H : Forall (λ x : A, R x x) (a :: l)
  x : A
  l0 : list A
  H2 : R a a
  H3 : Forall (λ x : A, R x x) l
  H0 : x = a
  H1 : l0 = l
  ============================
   (R a a)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = 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.
Level 1: Backtrace:

Tactic failure.