Walter Didimo Maurizio Pizzonia Upward Embeddings and Orientations of Undirected Planar Graphs. 221-241 2003 7 J. Graph Algorithms Appl. 2 http://www.cs.brown.edu/publications/jgaa/accepted/2003/DidimoPizzonia2003.7.2.pdf db/journals/jgaa/jgaa7.html#DidimoP03