Goal:
  
  T : Type -> Type
  M : Monad T
  A : Type
  P : A -> Type
  Q : A -> Type
  f : forall x : A, ∥ Q x ∥ -> T (∥ P x ∥)
  l0 : list A
  a : A
  l : list A
  allq : ∥ All Q (a :: l) ∥
  X : ∥ P a ∥
  Y : ∥ All P l ∥
  ============================
   (∥ All P (a :: l) ∥)


DEBUG:Going to execute:
fun H => let ph := fresh "H" in
                    destruct H as [H ph]