Remark. In the text on the abstracts server, the words “Prodanov’s proof” in the second paragraph must be replaced with “a proof given by Dimiter Vakarelov”, and the Internet link for the first item in the references section must be omitted. The link must be omitted also in the references section of the above-mentioned draft version of Skordev's paper. The version in question is obtained from the initial one (which is from March 1999) by the replacement indicated at the beginning of the remark, a small change in the last paragraph on page 4 made in connection with this replacement, a change in the Internet link that must be omitted now, by skipping an outdated link and by the correction of several misprints (in particular, a replacement of “Problem 3” with “Problem 2” was done in the last paragraph on page 4). The initial version is still accessible – as a DVI file and a Postscript file.