[Nauty] Bug in code for writting sparse6 format

[Brendan McKay]

Dear nauty people,

It gives me no pleasure to inform you that a bug has been lurking
for a long time without detection in the procedure that converts
graphs into sparse6 format.

The bug is moderately hard to invoke (which is probably why it went
unnoticed), and I'm hoping that nobody has been seriously bitten by
it. Sparse6 is not the default output format for any of the nauty
utilities, so unless you have been using it deliberately there is
no need for concern.  nauty itself is not effected.

The details can be found at
   http://cs.anu.edu.au/~bdm/nauty/s6bug.html
along with a utility by which you can check any sparse6 files you
happen to have.

I recommend that everyone install the corrected software.  You can
just download the replacement file gtools.c from the above page
and recompile, or you can replace the whole package.

Please let me know if you aren't sure whether you have problems
or if you need help in recovering.

My apologies.

Brendan.