@Override
    protected List<?> map(final Traverser.Admin<S> traverser) {
        final Collection listA = convertTraverserToCollection(traverser);
        final Collection listB = (null != valueTraversal) ? convertTraversalToCollection(traverser, valueTraversal) : convertArgumentToCollection(parameterItems.get());

        final List combined = new ArrayList(listA);
        combined.addAll(listB);

        return combined;
    }
A map step that returns the combination of the traverser and the provided arguments. This is also commonly known as
concatenation or append.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/CombineStep.java