code: def _load_bib_db(self):
          r
          # Get the names of custom bibtex files by parsing the
          # \bibliography command and filtering out the default lsstdoc
          # bibliographies.
          command = LatexCommand(
              'bibliography',
              {'name': 'bib_names', 'required': True, 'bracket': '{'})
          try:
              parsed = next(command.parse(self._tex))
              bib_names = [n.strip() for n in parsed['bib_names'].split(',')]
          except StopIteration:
              self._logger.warning('lsstdoc has no bibliography command')
              bib_names = []
          custom_bib_names = [n for n in bib_names
                              if n not in KNOWN_LSSTTEXMF_BIB_NAMES]
  
          # Read custom bibliographies.
          custom_bibs = []
          for custom_bib_name in custom_bib_names:
              custom_bib_path = os.path.join(
                  os.path.join(self._root_dir),
                  custom_bib_name + '.bib'
              )
              if not os.path.exists(custom_bib_path):
                  self._logger.warning('Could not find bibliography %r',
                                       custom_bib_path)
                  continue
              with open(custom_bib_path, 'r') as file_handle:
                  custom_bibs.append(file_handle.read())
          if len(custom_bibs) > 0:
              custom_bibtex = '\n\n'.join(custom_bibs)
          else:
              custom_bibtex = None
  
          # Get the combined pybtex bibliography
          db = get_bibliography(bibtex=custom_bibtex)
  
          self._bib_db = db
docs: r"""Load the BibTeX bibliography referenced by the document.
  
          This method triggered by the `bib_db` attribute and populates the
          `_bib_db` private attribute.
  
          The ``\bibliography`` command is parsed to identify the bibliographies
          referenced by the document.