@Override
    public void set(final String key, final Object value) throws IllegalArgumentException {
        throw TraversalSideEffects.Exceptions.sideEffectKeyDoesNotExist(key);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/EmptyTraversalSideEffects.java