% This material goes at the beginning of all Stanford GraphBase CWEB files\def\topofcontents{  \leftline{\sc\today\ at \hours}\bigskip\bigskip  \centerline{\titlefont\title}}\font\ninett=cmtt9\def\botofcontents{\vskip 0pt plus 1filll    \ninerm\baselineskip10pt    \noindent\copyright\ 1993 Stanford University    \bigskip\noindent    This file may be freely copied and distributed, provided that    no changes whatsoever are made. All users are asked to help keep    the Stanford GraphBase files consistent and ``uncorrupted,''    identical everywhere in the world. Changes are permissible only    if the modified file is given a new name, different from the names of    existing files in the Stanford GraphBase, and only if the modified file is    clearly identified as not being part of that GraphBase.    (The {\ninett CWEB} system has a ``change file'' facility by    which users can easily make minor alterations without modifying    the master source files in any way. Everybody is supposed to use    change files instead of changing the files.)    The author has tried his best to produce correct and useful programs,    in order to help promote computer science research,    but no warranty of any kind should be assumed.    \smallskip\noindent    Preliminary work on the Stanford GraphBase project    was supported in part by National Science    Foundation grant CCR-86-10181.}\def\prerequisite#1{\def\startsection{\noindent    Important: Before reading {\sc\title},    please read or at least skim the program for {\sc#1}.\bigskip    \let\startsection=\stsec\stsec}}\def\prerequisites#1#2{\def\startsection{\noindent    Important: Before reading {\sc\title}, please read    or at least skim the programs for {\sc#1} and {\sc#2}.\bigskip    \let\startsection=\stsec\stsec}}