add(greener => new UIState {
      on(mouse clicked) --> new UIState {
          implicit val twin = Linear(1 second)
          val init = mouse.position.copy
          val alpha = Cell(0.0) ~> 1
          val rect = Rectangle(init, init+(30,30))

          draw(rect fill Color(green, alpha))

          on (mouse released) --> new UIState {
              implicit val twin = Linear(1 second)
              draw(rect fill Color(green, Cell(!alpha) ~> 0))

              on (twin finished) --> greener()
          }
      }
  })