guava/src/com/google/common/collect/ImmutableSortedSet.java
public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E element) {
    return new RegularImmutableSortedSet<E>(ImmutableList.of(element), Ordering.natural());
  }
public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E element) {
    return new RegularImmutableSortedSet<E>(ImmutableList.of(element), Ordering.natural());
  }
Returns an immutable sorted set containing a single element.