Goal:
  
  A : Type
  P : nat → A → A → Type
  Q : A → Type
  hd : A
  tl : list A
  i : nat
  hd' : A
  p : P i hd hd'
  X : Q hd
  X0 : All Q tl
  ============================
   (P i hd hd')


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