Address for correspondence:

Hierarchical Systems Department
Institute of Computer and Communication Systems
Bulgarian Academy of Sciences
Acad. G. Bonchev str. Bl. 2, 1113 Sofia, Bulgaria
Phone: (+359 2) 979 27-74
Fax: (+359 2) 872 39 05
Web page: http://hsi.iccs.bas.bg
Person for contacts: Prof. D.Sc., PhD Todor Stoilov
E-mail: todor@hsi.iccs.bas.bg