The chemistry of bicyclic bisureas. 3. The synthesis and hydrolytic stability of the difluoroborate salts of 3,7-diethoxy-2,6-diaza-4,8-diazoniabicyclo[3.3.0]octane-3,7-dienes and 3,7-diethoxy-2,6-diaza-4,8-diazoniabicyclo[3.3.1]non-ane-3,7-dienes