A window that can expand to the right and contract from the left

window.scala

case class Window(l: Int, r: Int) {
  def size = r - l
  def ||->(i: Int): Window = this.copy(r = r + i) //widen by moving right edge right
  def |->|(i: Int): Window = this.copy(l = l + i) //shrink by moving left edge right
}