code: def _file_has_tag_anchor_keypair(self, anchors, file_key, tag):
          
          return file_key in anchors and tag in anchors[file_key]
docs: Is there an AnchorHub tag, 'tag', registered for file 'file_key' in
          'anchors'?
  
          :param anchors: Dictionary mapping string file paths to inner
              dictionaries. These inner dictionaries map string AnchorHub tags
              to string generated anchors
          :param file_key: The absolute path to the file that may or may not
              have the AnchorHub tag in it. Used as a key to anchors
          :param tag: The string being tested
          :return: True if tag is a valid AnchorHub tag in the file associated
              with 'file_key'