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


DEBUG:Going to execute:
intro H