human
Solve the following python programming problem given the following code:
def _render_view(self, req, milestone) : 
    milestone_groups = [] 
    available_groups = [] 
    default_group_by_available = False 
    ticket_fields = TicketSystem (self.env).get_ticket_fields () 
    for field in ticket_fields : 
        if (((field ['type'] == 'select') and (field ['name'] != 'milestone')) or (field ['name'] in ('owner', 'reporter'))) : 
            available_groups.append ({ 
                'name' : field ['name'], 
                'label' : field ['label'], 
}) 
            if (field ['name'] == self.default_group_by) : 
                default_group_by_available = True 
by = None 
    if <MASK> : 
        by = self.default_group_by 
else : 
        if available_groups : 
            by = available_groups [0] ['name'] 
by = req.args.get ('by', by) 
    tickets = get_tickets_for_milestone (self.env, milestone = milestone.name, field = by) 
    tickets = apply_ticket_permissions (self.env, req, tickets) 
    stat = get_ticket_stats (self.stats_provider, tickets) 
    context = web_context (req, milestone.resource) 
    data = { 
        'context' : context, 
        'milestone' : milestone, 
        'attachments' : AttachmentModule (self.env).attachment_data (context), 
        'available_groups' : available_groups, 
        'grouped_by' : by, 
        'groups' : milestone_groups, 
} 
    data.update (milestone_stats_data (self.env, req, stat, milestone.name)) 
    if by : 
        def per_group_stats_data(gstat, group_name) : 
            return milestone_stats_data (self.env, req, gstat, milestone.name, by, group_name) 
milestone_groups.extend (grouped_stats_data (self.env, self.stats_provider, tickets, by, per_group_stats_data)) 
add_stylesheet (req, 'common/css/roadmap.css') 
    add_script (req, 'common/js/folding.js') 
    def add_milestone_link(rel, milestone) : 
        href = req.href.milestone (milestone.name, by = req.args.get ('by')) 
        add_link (req, rel, href, _ ('Milestone "%(name)s"', name = milestone.name)) 
milestones = [m for m in Milestone.select (self.env)...
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
default_group_by_available