* the result set is constructed, the input set is merely copied. Only as the result set is
   * iterated are the individual subsets created. Each of these subsets occupies an additional O(n)
   * memory but only for as long as the user retains a reference to it. That is, the set returned by
   * {@code combinations} does not retain the individual subsets.
   *
   * @param set the set of elements to take combinations of

   * meet the contract guaranteed by this method.
   *
   * <p>There is no {@link Iterable} equivalent to this method, so use this method to wrap each
   * individual iterator as it is generated.
   *
   * @param iterator the backing iterator. The {@link PeekingIterator} assumes ownership of this
   *     iterator, so users should cease making direct calls to it after calling this method.

   * meet the contract guaranteed by this method.
   *
   * <p>There is no {@link Iterable} equivalent to this method, so use this method to wrap each
   * individual iterator as it is generated.
   *
   * @param iterator the backing iterator. The {@link PeekingIterator} assumes ownership of this
   *     iterator, so users should cease making direct calls to it after calling this method.