/**
 * GWT emulated version of {@link com.google.common.collect.ImmutableSet}. For the unsorted sets,
 * they are thin wrapper around {@link java.util.Collections#emptySet()}, {@link
 * Collections#singleton(Object)} and {@link java.util.LinkedHashSet} for empty, singleton and
 * regular sets respectively. For the sorted sets, it's a thin wrapper around {@link
 * java.util.TreeSet}.
 *
 * @see ImmutableSortedSet
 * @author Hayward Chan
 */