code: def with_more_selectors(self, selectors):
          
          if self.headers and self.headers[-1].is_selector:
              new_selectors = extend_unique(
                  self.headers[-1].selectors,
                  selectors)
              new_headers = self.headers[:-1] + (
                  BlockSelectorHeader(new_selectors),)
              return RuleAncestry(new_headers)
          else:
              new_headers = self.headers + (BlockSelectorHeader(selectors),)
              return RuleAncestry(new_headers)
docs: Return a new ancestry that also matches the given selectors.  No
          nesting is done.