* of each element is the <i>maximum</i> of its counts in the two backing multisets. The iteration
   * order of the returned multiset matches that of the element set of {@code multiset1} followed by
   * the members of the element set of {@code multiset2} that are not contained in {@code
   * multiset1}, with repeated occurrences of the same element appearing consecutively.
   *

   * sets ("constant sets"), with a given comparator. Example:
   *
   * <pre>{@code
   * public static final ImmutableSortedSet<Number> LUCKY_NUMBERS =
   *     new ImmutableSortedSet.Builder<Number>(ODDS_FIRST_COMPARATOR)
   *         .addAll(SINGLE_DIGIT_PRIMES)
   *         .add(42)
   *         .build();
   * }</pre>
   *