Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  X : All P l
  X0 : ∀ x : A, P x → Q x
  X1 : All Q l
  ============================
   (All Q l)


DEBUG:Going to execute:
PTRDEBUGTACmatch goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end