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:
constructor 1
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := p (f a) && forallb p (map f l) =
      p (f a) && forallb (λ x : A, p (f x)) l
of type
constr of type tacvalue