Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  HPQ : ∀ x y : A, Q x → P x y → Q y
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  q : Q hd
  H : All Q tl
  ============================
   (Q ?x)


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