@Override
    public void addStart(final Traverser.Admin<Vertex> start) {
        final VertexProperty<TraverserSet<Object>> property = start.get().<TraverserSet<Object>>property(TraversalVertexProgram.HALTED_TRAVERSERS);
        this.count = property.isPresent() ? property.value().bulkSize() : 0l;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/traversal/lambda/HaltedTraversersCountTraversal.java