code: public void addDerivedSipSessions(MobicentsSipSession derivedSession) {
  		if(derivedSipSessions == null) {
  			this.derivedSipSessions = new ConcurrentHashMap<String, MobicentsSipSession>();
  		}
  		derivedSipSessions.putIfAbsent(derivedSession.getKey().getToTag(), derivedSession);
  	}
docs: /*
  (non-Javadoc)
  @see org.mobicents.servlet.sip.core.session.MobicentsSipSession#addDerivedSipSessions(org.mobicents.servlet.sip.core.session.MobicentsSipSession)