Pier Francesco Cortese Giuseppe Di Battista Fabrizio Frati Maurizio Patrignani Maurizio Pizzonia C-Planarity of C-Connected Clustered Graphs. 225-262 2008 12 J. Graph Algorithms Appl. 2 http://jgaa.info/accepted/2008/Cortese+2008.12.2.pdf db/journals/jgaa/jgaa12.html#CorteseBFPP08