*
   * <p><i>Notes:</i> This is an implementation of the Plain Changes algorithm for permutations
   * generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2.
   *
   * <p>If the input list contains equal elements, some of the generated permutations will be equal.
   *
   * <p>An empty collection has only one permutation, which is an empty list.
   *

   *
   * <p><i>Notes:</i> This is an implementation of the Plain Changes algorithm for permutations
   * generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2.
   *
   * <p>If the input list contains equal elements, some of the generated permutations will be equal.
   *
   * <p>An empty collection has only one permutation, which is an empty list.
   *