'Render the group header (=group label)
@param tbody: the TBODY or TABLE to append to
@param group: the group dict
@param level: the grouping level'

def html_render_group_header(self, tbody, group, level=0):

    data = self.data
    columns = data.get('c')
    value = group.get('v')
    if (not value):
        value = ''
    header = TD(s3_unicode(s3_strip_markup(value)), _colspan=(len(columns) if columns else None))
    tbody.append(TR(header, _class=('gi-group-header    gi-level-%s' % level)))
