title: Depth first search tree edge classification
body: <p>I've got a basic question about DFS edge classification:  I have a directed graph with edges: 1->2, 2->3 and 1->3.  The edge classification for 1->2 is tree edge, 2->3 is tree edge.  I'm confused as to what the classification of 1->3 would be: forward edge, back edge or tree edge?</p>