Volker Diekert Manfred Kufleitner On First-Order Fragments for Words and Mazurkiewicz Traces. 1-19 2007 conf/dlt/2007 Developments in Language Theory http://dx.doi.org/10.1007/978-3-540-73208-2_1 db/conf/dlt/dlt2007.html#DiekertK07