Goal:
  
  A : Type
  P : A → Type
  p : A → bool
  x : A
  l : list A
  p0 : P x
  X : All P l
  IHX : ∀ p : A → bool, (∀ x : A, P x → p x) → forallb p l
  H : ∀ x : A, P x → p x
  ============================
   (is_true (p x))


DEBUG:Going to execute:
F x2 x1 x0
Level 0: Backtrace:

P is used in hypothesis p0.
Level 0: Backtrace:

P is used in hypothesis p0.
Level 0: Backtrace:

P is used in hypothesis p0.
Level 0: Backtrace:

P is used in hypothesis p0.
Level 0: Backtrace:

P is used in hypothesis p0.
Level 1: Backtrace:

P is used in hypothesis p0.
Level 0: Backtrace:

P is used in hypothesis p0.