Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  x : A
  xs : list A
  y : C
  h : ∥ All (P y) (x :: xs) ∥
  X : Q y
  ============================
   (∥ All (P y) xs ∥)


DEBUG:Going to execute:
case H; clear H