static <E> Sets.SetView<E> 
Sets.difference(Set<E> set1,
Set<?> set2) 
Returns an unmodifiable view of the difference of two sets.

static <E> Sets.SetView<E> 
Sets.intersection(Set<E> set1,
Set<?> set2) 
Returns an unmodifiable view of the intersection of two sets.

static <E> Sets.SetView<E> 
Sets.symmetricDifference(Set<? extends E> set1,
Set<? extends E> set2) 
Returns an unmodifiable view of the symmetric difference of two sets.

static <E> Sets.SetView<E> 
Sets.union(Set<? extends E> set1,
Set<? extends E> set2) 
Returns an unmodifiable view of the union of two sets.
