Lars Grunske Kirsten Winter Nisansala Yatapanage Defining the abstract syntax of visual languages with advanced graph grammars - A case study based on behavior trees. 343-379 2008 19 J. Vis. Lang. Comput. 3 http://dx.doi.org/10.1016/j.jvlc.2007.11.003 db/journals/vlc/vlc19.html#GrunskeWY08