This structure contains additional information for an edge of an undi_graph class object. In particular, it contains:
1 - a non-negative integer identifier for the edge;
2 - a "source" gdtnode;
3 - a "target" gdtnode;
4 - a "start" gdtnode if the edge is directed (if the edge is not directed, this field is equal to NULL_EDGE);
5 - the positions of the edge in all the adjacent lists of its end-gdtnodes;
6 - a list of all markers of the edge;
7 - a list of all constraints involving the edge.