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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _Notify(username, notification_type, message, object_reference):
 """Schedules a new-style REL_DB user notification."""


  # Do not try to notify system users (e.g. Cron).
  if username in aff4_users.GRRUser.SYSTEM_USERS:
    return

  if object_reference:
    uc = object_reference.UnionCast()
    if hasattr(uc, "client_id"):
      message = _HostPrefix(uc.client_id) + message

  n = rdf_objects.UserNotification(
      username=username,
      notification_type=notification_type,
      state=rdf_objects.UserNotification.State.STATE_PENDING,
      message=message,
      reference=object_reference)
  data_store.REL_DB.WriteUserNotification(n)