guava/src/com/google/common/collect/ImmutableSortedSet.java
@SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2) {
    return construct(Ordering.natural(), 2, e1, e2);
  }
@SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2) {
    return construct(Ordering.natural(), 2, e1, e2);
  }
Returns an immutable sorted set containing the given elements sorted by their natural ordering.
When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
one specified is included.

@throws NullPointerException if any element is null