Goal:
  
  A : Type
  P : A → Prop
  p : A → bool
  a : A
  l : list A
  Hp : ∀ x : A, reflect (P x) (p x)
  IHl : forallb p l → Forall P l
  pa : is_true false
  pl : is_true (forallb p l)
  n : ¬ P a
  ============================
   (P a)


DEBUG:Going to execute:
symmetry; trivial
Level 1: Backtrace:

Cannot find a relation to rewrite.
Level 0: Backtrace:

Cannot find a relation to rewrite.