Grädel E. Finite Model Theory and Its Applications / by Erich Grädel, Phokion G. Kolaitis, Leonid Libkin, Maarten Marx, Joel Spencer, Moshe Y. Vardi, Yde Venema, Scott Weinstein. // Springer e-books. URL: http://dx.doi.org/10.1007/3-540-68804-8