code: public void addTangoQualityChangeListener(ITangoQualityChangeListener listener, String attrName, String[] filters, boolean stateless)
              throws DevFailed {
          TangoQualityChange tangoQualityChange;
          String key = deviceName+"/"+attrName;
          if ((tangoQualityChange = tango_quality_change_source.get(key)) == null) {
              tangoQualityChange = new TangoQualityChange(deviceProxy, attrName, filters);
              tango_quality_change_source.put(key, tangoQualityChange);
          }
  
          synchronized (moni) {
              tangoQualityChange.addTangoQualityChangeListener(listener, stateless);
          }
      }
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