[web2py] button action

[niknok]

I have this button that redirects to a URL:

form[0][-1][1].append(INPUT(_type='button',_value=T('Previous')
                            ,_onclick='document.location="%s"' 
%URL("question")))

what do I add to _onclick to do the following:
1) session.current_item -=1
2) redirect(URL('question'))

/r
Nik