Goal:
  
  A : Type
  p : A → bool
  x : A
  l : list A
  H : is_true (p x)
  H0 : Forall (λ x : A, p x) l
  IHForall : is_true (forallb p l)
  ============================
   (p x && forallb p l = true)


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.