It's not specifically related to the original article discussed here, but skiplists don't need to be unidirectional. For instance Redis implements all the sorted sets operations using doubly-linked skiplists, so once you identify a node you can traverse other nodes in both directions.
Good point- ours are currently unidirectional because they have to be lock free. We have some ideas on how to keep them lock free and make them bidirectional, but that's not part of the current release.
