How to tick checkboxes with the same name?

[malayrev at gmail.com]

I faced a problem: to implement appropriate search program I need to tick few checkboxes which turned out to have the same name (name="a", id="a1","a2","a3","a4"). Set_input('a', True) does not work (I use Grab library), this command leads to the error "checkboxgroup must be set to a sequence". I don't understand what the sequence actually is, so I'm stuck with how to tick the checkboxes. It would be really great if someone would help me with how to handle this question. The code is available here:
view-source:http://zakupki.gov.ru/pgz/public/action/contracts/search/ext/enter