Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  p : A → bool
  IHl : ∀ p : A → bool, forallb p l → (∀ x : A, p x → P x) → Forall P l
  H0 : ∀ x : A, p x → P x
  H1 : is_true (p a)
  H2 : is_true (forallb p l)
  ============================
   (Forall P l)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
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.