@misc{gdea_3596,
          editor = {J\'anos Pach},
           title = {Building Blocks of Upward Planar Digraphs},
          author = {Patrick Healy and Karol Lynch},
       publisher = {Springer},
            year = {2004},
           pages = {296--306},
             url = {http://gdea.informatik.uni-koeln.de/596/},
        abstract = {We show that a digraph is upward planar if and only if its biconnected components have certain properties.
}
}