/**
     * the entry point we started building the graph from
     */
    MetadataGraphVertex entry;

    // graph vertices
    TreeSet<MetadataGraphVertex> vertices;

    /**
     * incident and excident edges per node
     */
    Map<MetadataGraphVertex, List<MetadataGraphEdge>> incidentEdges;

    Map<MetadataGraphVertex, List<MetadataGraphEdge>> excidentEdges;

    /**
     *  null in dirty graph, actual