@Override
    public <S, C> List<Traversal.Admin<S, C>> getLocalChildren() {
        final List<Traversal.Admin<S, C>> children = new ArrayList<>();
        if (mergeTraversal != null) children.add((Traversal.Admin<S, C>) mergeTraversal);
        if (onMatchTraversal != null) children.add((Traversal.Admin<S, C>) onMatchTraversal);
        if (onCreateTraversal != null) children.add((Traversal.Admin<S, C>) onCreateTraversal);
        return children;
    }
Determine if this is the first pass through {@link #processNextStart()}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeElementStep.java