% 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\ 1992 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}}