Collapsed \cite{this}, \cite{that}, \cite{more} into \cite{this,that,more}.


The following references will not appear because they were not cited in the text.

%  \bibitem{3} J.-P. Borel and F. Laubie, Construction de mots de Christoffel, \emph{C. R. Acad. Sci. Paris (I)} \textbf{313} (1991), 483--485.
%  \bibitem{4} J.-P. Borel and F. Laubie, Quelques mots sur la droite projective reelle, \emph{Seminaire de Theorie des Nobres de Bordeaux}, to appear.
%  \bibitem{40} E. C. Zeeman, An algorithm for Eudoxan and anthipairetic ratios, preprint.