@Article{FerSti_j2001_a, author = {Henning~Fernau and Ralf~Stiebe}, title = {Valences in Lindenmayer Systems}, journal = fundinform, year = 2001, volume = {45}, number = {4}, pages = {329--358}, url = {http://iospress.metapress.com/content/dgltqxnxpdhanexm/fulltext.pdf} } %http://fi.mimuw.edu.pl/abs45.html#17