Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  l' : list A
  H0 : P a
  H1 : Forall P (l ++ l')
  H2 : Forall P l
  H3 : Forall P l'
  ============================
   (Forall P (a :: l))


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 := Forall P (a :: l)
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.