The author is indebted to P. Maass and W. Dieterich from the Universität Konstanz;
R. Hempelmann from the Universität Saarbrücken;
and Izabela Sosnowska from the Warsaw University for stimulating discussions. This work was partially supported by the Polish KBN Grant No. 2 P03B 059 12 and the Deutsche Forschungsgemainschaft-Polish Academy of Sciences Project No. 436 POL 113/85/0;