class_docstr: Basic in-memory dict-based object for saving Task Records.
  
  This is the first object to present the DB interface
  for logging tasks out of memory.
  
  The interface is based on MongoDB, so adding a MongoDB
  backend should be straightforward.
docstr: cull the oldest N records
docstr_tok: ["cull","the","oldest","N","records"]
code: def _cull_oldest(self, n=1):
      """cull the oldest N records"""
      for msg_id in self.get_history()[:n]:
          self.log.debug("Culling record: %r", msg_id)
          self._culled_ids.add(msg_id)
          self.drop_record(msg_id)
  
imports: import copy
  from copy import deepcopy
  from datetime import datetime
  from traitlets import Dict, Float, Integer, Unicode
  from traitlets.config.configurable import LoggingConfigurable
  from ..util import ensure_timezone
label_desc: Use image node_id 6 for calling the DictDB obj's underlying member method code with example usage: obj._cull_oldest(n) without return types