sortedVertices.add(new LinkedList<Vertex>());
for (Iterator<Vertex> iV = verticesLeft.iterator(); iV.hasNext(); ){
v = iV.next();
// find starting vertices
if (v.inDegree() == 0) {
startingVertices.add(v);
iV.remove();
}
// find ending vertices
else if (v.outDegree() == minimalOutDegree) {