code: public final Set<V> getValues(K key) {
  	Set<V> set = _getValues(key);
  	if(set == null)
  	    return Collections.emptySet();
  	else 
  	    return Collections.unmodifiableSet(set);
      }
docs: Returns the image of <code>key</code> through this relation.
  The returned collection IS IMMUTABLE.
  Returns <code>Collections.emptySet()</code> if no value is attached to <code>key</code>.