@ARTICLE{DasPauThiYu_j2001, AUTHOR = {J{\"u}rgen~Dassow and Gheorghe~P\u{a}un and Gabriel~Thierrin and Sheng~Yu}, TITLE = {Tree systems of morphisms}, JOURNAL = actainform, YEAR = 2001, VOLUME = 38, NUMBER = 2, PAGES = {131--153}, url = {http://www.springerlink.com/content/r4e38klcnv940bb2/fulltext.pdf} }