public default Set<String> keys() {
        final Set<String> keys = new HashSet<>();
        this.properties().forEachRemaining(property -> keys.add(property.key()));
        return Collections.unmodifiableSet(keys);
    }
Get the keys of the properties associated with this element.
The default implementation iterators the properties and stores the keys into a {@link HashSet}.

@return The property key set
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Element.java