'Outputs a <ul> for this set of choice fields.
If an id was given to the field, it is applied to the <ul> (each
item in the list will get an id of `$id_$i`).'

def render(self):

    id_ = self.attrs.get(u'id', None)
    start_tag = (format_html(u'<ul    id="{0}">', id_) if id_ else u'<ul>')
    output = [start_tag]
    for widget in self:
        output.append(format_html(u'<li>{0}</li>', force_text(widget)))
    output.append(u'</ul>')
    return mark_safe(u'\n'.join(output))
