bug#10366: hello, i have a problem when i use chmod, can u explan why?thks

[lidong_bit]

in /var/, i have a directory:www, its property is:
d-w-------  2 root root  4096 2011-12-26 01:24 www
i want to change the property so that all the user have the write authority:
1.
address@hidden:/var$ sudo chmod a +w -R ./www/
chmod: invalid mode: `a'
Try `chmod --help' for more information.
address@hidden:/var$

2.
address@hidden:/var$ sudo chmod o +w -R ./www/
chmod: invalid mode: `o'
Try `chmod --help' for more information.
address@hidden:/var$


I don't why this happens, is it a buy? or is it forbidden?