public IEnumerable<CEDescr<VProperties, EProperties>> GetInEdges(CVDescr<VProperties> v)
            {
                Debug.Assert(v.reference is LinkedListNode<SRodVertexPair>);

                var preds = (v.reference as LinkedListNode<SRodVertexPair>).Value.preds;
                foreach (CEDescr<VProperties, EProperties> ed in preds)
                {
                    yield return ed;
                }
            }