code: public static <T> Set<T> newUnmodifiableSet(Collection<T> contents)
  	{
  		if(isEmpty(contents))
  			return Collections.emptySet();
  		if(contents.size() == 1)
  			return Collections.singleton(contents.iterator().next());
  		return Collections.unmodifiableSet(dup(contents));
  	}
docs: Create a unmodifiable set with contents.
  @param contents The contents of the set.
  @return A unmodifiable set containing contents.