Frank Drewes Christine du Toit Sigrid Ewert Brink van der Merwe Andries P. J. van der Walt Bag Context Tree Grammars. 459-480 2008 86 Fundam. Inform. 4 http://iospress.metapress.com/content/13k8650405260830/ db/journals/fuin/fuin86.html#DrewesTEMW08