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


DEBUG:Going to execute:
match goal with
           | H:∥ _ ∥ |- _ => case H; clear H; intro H
           end