@Override
    public <S, E> List<Traversal.Admin<S, E>> getLocalChildren() {
        final List<Traversal.Admin<S, E>> children = new ArrayList<>();
        if (mapTraversal != null) children.add((Traversal.Admin<S, E>) mapTraversal);
        children.addAll(parameters.getTraversals());
        return children;
    }
Reference implementation for service calls via the {@link ServiceRegistry} and {@link Service} APIs. This step
can be used to start a traversal or it can be used mid-traversal with one at a time or barrier execution.

@author Mike Personick (http://github.com/mikepersonick)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/CallStep.java