Goal:
  
  A : Type
  P : A → Type
  x : A
  l : list A
  p : ∥ P x ∥
  X : All (λ x : A, ∥ P x ∥) l
  IHX : ∥ All P l ∥
  ============================
   (∥ All P (x :: l) ∥)


DEBUG:Going to execute:
case H; clear H; intro H