Javadoc
Returns a set view of the values contained in this BidiMap. The set is
backed by the map, so changes to the map are reflected in the collection,
and vice-versa. If the map is modified while an iteration over the
collection is in progress (except through the iterator's own
remove operation), the results of the iteration are undefined.
The collection supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the add or addAll
operations.