code: def _open_list(self, list_type):
          
          if list_type in LIST_TYPES.keys():
              tag = LIST_TYPES[list_type]
          else:
              raise Exception('CustomSlackdownHTMLParser:_open_list: Not a valid list type.')
  
          html = '<{t} class="list-container-{c}">'.format(
              t=tag,
              c=list_type
          )
          self.cleaned_html += html
          self.current_parent_element['tag'] = LIST_TYPES[list_type]
          self.current_parent_element['attrs'] = {'class': list_type}
docs: Add an open list tag corresponding to the specification in the
          parser's LIST_TYPES.