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


DEBUG:Going to execute:
match H with
           | H' => fail 2
           | _ => unify X Y; clear H' || clear H
           end