new_contents: from active_redis.core import Observable
  from active_redis.registry import Observable as Registry
  
  @Registry.register
  class List(Observable):
    """List observable."""
    type = list
    watch_methods = [
      '__setitem__',
      'append',
      'extend',
      'insert',
      'remove',
      'pop',
    ]
  
  @Registry.register
  class Dict(Observable):
    """Dictionary observable."""
    type = dict
    watch_methods = [
      '__setitem__',
    ]
  
subject: Add observables.