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)
  ============================
   (P a)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := a
of type
intropattern
id0 := p
of type
intropattern
flags := 
of type
tauto_flags
X2 := bool
of type
constr of type tacvalue