@Override
        public boolean equals(final Object o) {
            return o instanceof IndexedNull;
        }
Provides a way for an index to have a {@code null} value as {@code ConcurrentHashMap} will not allow a
{@code null} key.
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/structure/AbstractTinkerIndex.java