1. A.P. Ershov Institute of Informatics Systems, SB RAS, 6, Acad. Lavrentiev av., Novosibirsk, 630090, Russia. natamosk@ngs.ru
2. A.P. Ershov Institute of Informatics Systems, SB RAS, 6, Acad. Lavrentiev av., Novosibirsk, 630090, Russia, and Novosibirsk State University, 2, Pirogova st., Novosibirsk, 630090, Russia. virb@iis.nsk.su