Nature 369, 67–71 (1994)

A Correction to this Letter was published on 11 December 1997 (Nature 390, 643; 1997), submitted by one of the authors (G.B.). However, because of an error in the editorial office, the correction had bypassed our standard procedures of checking with all authors and, where differences arise, with peer reviewers. The Correction therefore should not have been published. It has since become clear that the matter is one of scientific disagreement between G.B. and the other authors, who stand by their results in the original paper.