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;
}
}