docs: def as_checks(self, tmpl=TMPL, _items=None, **kwargs):
          """Render the field as a series of checkboxes, using the `tmpl`
          parameter as the template for each item.
  
          :param tmpl:
              HTML template to use for rendering each item.
  
          :param **kwargs:
              Named paremeters used to generate the HTML attributes of each item.
              It follows the same rules as `get_html_attrs`
  
          """
          attrs = self.extra.copy()
          attrs.update(kwargs)
          attrs['type'] = 'checkbox'
          attrs['name'] = self.name
          html = []
          tmpl = to_unicode(tmpl)
          values = self.to_string(**attrs) or []
          items = _items or self.items
  
          for item in items:
              if isinstance(item, list):
                  html.extend(self._render_fieldset(item, attrs, values, tmpl))
              else:
                  html.append(self._render_item(item, attrs, values, tmpl))
          return Markup('\n'.join(html))