class_docstr: Common part for ConditionalRegistry and MergedRegistry.
code: def __init__(self):
      # `Registry` to be synchronized with all the others.
      self._registry2 = Registry()
      self._last_version = None
  
      # The 'extra' registry. Mostly for backwards compatibility.
      self._extra_registry = Registry()
  
imports: from __future__ import unicode_literals
  from abc import ABCMeta, abstractmethod
  from prompt_toolkit.cache import SimpleCache
  from prompt_toolkit.filters import CLIFilter, to_cli_filter, Never
  from prompt_toolkit.keys import Key, Keys
  from six import text_type, with_metaclass
label_desc: Use image node_id 1 to create a new _AddRemoveMixin object from inherited base classes: BaseRegistry with example: obj = _AddRemoveMixin()