Stephan Kreutzer Martin Otto Nicole Schweikardt Boundedness of Monadic FO over Acyclic Structures. 571-582 2007 conf/icalp/2007 ICALP http://dx.doi.org/10.1007/978-3-540-73420-8_50 db/conf/icalp/icalp2007.html#KreutzerOS07