code: func swap(u, w Comparators, index int) {
  	for i := index; i < len(u); i++ {
  		u[i], w[i-index] = w[i-index], u[i]
  	}
  }
docs: // swap will swap positions of the two lists from index
  // to the end of the list.  It expects that these lists
  // are the same size or one different.