Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  X : All P l
  IHX : All (λ x : A, P x → Q x) l → All Q l
  X0 : All (λ x : A, P x → Q x) (x :: l)
  x0 : A
  l0 : list A
  X2 : All (λ x : A, P x → Q x) l
  H0 : x0 = x
  H1 : l0 = l
  X3 : Q x
  ============================
   (Q x)


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