Goal:
  
  A : Type
  P : nat → A → Type
  Q : nat → A → Type
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  X0 : ∀ (n : nat) (x : A), P n x → Q n x
  X1 : Alli Q (S n) tl
  ============================
   (Q n hd)


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