How hard is it to determine if a graph has a 2-role assignment?

Fred S. Roberts Li Sheng How hard is it to determine if a graph has a 2-role assignment? 67-73 2001 37 Networks 2 http://dx.doi.org/10.1002/1097-0037(200103)37:2<67::AID-NET1>3.0.CO;2-9 db/journals/networks/networks37.html#RobertsS01