derived from but not quite compatible with that of sgmllib, so it's a
new file.  I suppose it needs documentation, and htmllib needs to be
changed to use this instead of sgmllib, and sgmllib needs to be
declared obsolete.  But that can all be done later.

This code was first published as part of TAL (part of Zope Page
Templates), but that was strongly based on sgmllib anyway.  Authors
are Fred drake and Guido van Rossum.