def is_member(self, ldap_user, group_dn):
return None<EOL>
This method is an optimization for determining group membership without
loading all of the user's groups. Subclasses that are able to do this
may return True or False. ldap_user is as above. group_dn is the
distinguished name of the group in question.

The base implementation returns None, which means we don't have enough
information. The caller will have to call user_groups() instead and look
for group_dn in the results.