1. A.P. Ershov Institute of Informatics Systems, SB RAS, 6, Acad. Lavrentiev avenue, 630090, Novosibirsk, Russia
2. Sobolev Institute of Mathematics, SB RAS, 4, Acad. Koptyug avenue, 630090, Novosibirsk, Russia
3. A.P. Ershov Institute of Informatics Systems, SB RAS, 6, Acad. Lavrentiev avenue, 630090, Novosibirsk, Russia. virb@iis.nsk.su