Víctor Dalmau Andrei A. Krokhin Benoit Larose First-order Definable Retraction Problems for Posets and Reflexive Graphs. 31-51 2007 17 J. Log. Comput. 1 http://dx.doi.org/10.1093/logcom/exl014 db/journals/logcom/logcom17.html#DalmauKL07