docs: def log(self, elem):
          '''
          .. seealso:: :attr:`log`
          '''
  
          if elem:
              self.__meta['log'].update({get_timestamp(precice=True): elem})
          mfile = self.__meta['header']['stage']
  
          self.__lock.acquire()
          try:
              j = read_json(mfile)
              if j != self.__meta:
                  write_json(mfile, self.__meta)
          finally:
              self.__lock.release()