@Override
    protected Traverser.Admin<E> processNextStart() {
        // when it's a start step a traverser needs to be created to kick off the traversal.
        if (isStart && first) {
            first = false;
            final TraverserGenerator generator = this.getTraversal().getTraverserGenerator();
            final Traverser.Admin<S> traverser = generator.generate(UNION_STARTER, (Step) this, 1L);

            // immediately drop the path which would include "false" otherwise. let the path start with the
            // traversers produced from the children to the union()
            traverser.dropPath();
            this.addStart(traverser);
        }

        // if this isStart then loop through the next starts until we find one that doesn't hold the
        // UNION_STARTER and return that. this deals with inject() which is always a start step even
        // when used mid-traversal.
        Traverser.Admin<E> t = super.processNextStart();
        if (isStart) {
            while (t != null && t.get() == UNION_STARTER) {
                t = super.processNextStart();
            }
        }

        return t;
    }
A placeholder object used to kick off the first traverser in the union.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/branch/UnionStep.java