/** INTERNAL: Acquires a special historical session for reading objects as of a past time. */
  public org.eclipse.persistence.sessions.Session acquireHistoricalSession(AsOfClause clause)
      throws ValidationException {
    if (isServerSessionBroker()) {
      throw ValidationException.cannotAcquireHistoricalSession();
    }

    // ? logMessage("acquire_client_session_broker", (Object[])null);
    SessionBroker historicalBroker = copySessionBroker();
    String sessionName;
    AbstractSession session;
    Iterator names = this.getSessionsByName().keySet().iterator();
    while (names.hasNext()) {
      sessionName = (String) names.next();
      session = getSessionForName(sessionName);
      historicalBroker.registerSession(sessionName, session.acquireHistoricalSession(clause));
    }
    return historicalBroker;
  }