code: public void addTangoChangeListener(ITangoChangeListener listener, String attrName, String[] filters, boolean stateless)
              throws DevFailed {
          TangoChange tangoChange;
          String key = deviceName+"/"+attrName;
          if ((tangoChange = tango_change_source.get(key)) == null) {
              tangoChange = new TangoChange(deviceProxy, attrName, filters);
              tango_change_source.put(key, tangoChange);
          }
  
          synchronized (moni) {
              tangoChange.addTangoChangeListener(listener, stateless);
          }
      }
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