def _parse_doc_ref(self):
command = LatexCommand(<EOL>'<STR_LIT>',<EOL>{'<STR_LIT:name>': '<STR_LIT>', '<STR_LIT>': True, '<STR_LIT>': '<STR_LIT:{>'})<EOL>try:<EOL><INDENT>parsed = next(command.parse(self._tex))<EOL><DEDENT>except StopIteration:<EOL><INDENT>self._logger.warning('<STR_LIT>')<EOL>self._handle = None<EOL>self._series = None<EOL>self._serial = None<EOL>return<EOL><DEDENT>self._handle = parsed['<STR_LIT>']<EOL>try:<EOL><INDENT>self._series, self._serial = self._handle.split('<STR_LIT:->', <NUM_LIT:1>)<EOL><DEDENT>except ValueError:<EOL><INDENT>self._logger.warning('<STR_LIT>'<EOL>'<STR_LIT>', self._handle)<EOL>self._series = None<EOL>self._serial = None<EOL><DEDENT>
Parse the document handle.

        Sets the ``_series``, ``_serial``, and ``_handle`` attributes.