Goal:
  
  A : Type
  B : Type
  f : A → B
  a : A
  l : list A
  p : B → bool
  IHl : ∀ (f : A → B) (p : B → bool),
          forallb p (map f l) = forallb (λ x : A, p (f x)) l
  ============================
   (p (f a) && forallb p (map f l) = p (f a) && forallb (λ x : A, p (f x)) l)


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