Deal with cross-edges

[michielbdejong]

In this graph, node C would falsely conclude it found a cycle (it should mark the treeToken as already backtracked):

a -> b  -> c
|          ^
 \__ d ___/

[michielbdejong]

If DDCD did its work correctly, DFS will never backtrack. Will need to construct a test where a link disappears early enough to trigger a backtrack, but late enough so not the whole tree crumbles down.

[michielbdejong]

http://www.geeksforgeeks.org/detect-cycle-direct-graph-using-colors/ distinguishes between tree edges (the ones visited first), back-edges (ones that close a cycle, given the tree edges), cross-edges (connects two nodes with common ancestors), and forward edges (from ancestor to descendant).

[michielbdejong]

Maybe it's simpler and safer never to try more than one sibling out-neighbor, see ledgerloops/ledgerloops-whitepaper#4