code: def _advance_new_study_id(self):
          
          c = self._next_study_id
          self._next_study_id = 1 + c
          content = u'{"next_study_id": %d}\n' % self._next_study_id
          # The content is JSON, but we hand-rolled the string above
          #       so that we can use it as a commit_msg
          self._write_master_branch_resource(content,
                                             self._id_minting_file,
                                             commit_msg=content,
                                             is_json=False)
          return c
docs: ASSUMES the caller holds the _doc_counter_lock !
          Returns the current numeric part of the next study ID, advances
          the counter to the next value, and stores that value in the
          file in case the server is restarted.