public static <K, V> Set<V> get(final Map<K, Set<V>> map, final K key) {
        final Set<V> set = getMapSet(map, key);
        return set == null ? Collections.emptySet() : set;
    }
A number of static methods to interact with a multi map, i.e. a map that maps keys to sets of values.

@author Matthias Broecheler (me@matthiasb.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/util/MultiMap.java