code: public void removeTangoPeriodicListener(ITangoPeriodicListener listener, String attrName)
              throws DevFailed {
          synchronized (moni) {
              TangoPeriodic tangoPeriodic;
              String key = deviceName+"/"+attrName;
              if ((tangoPeriodic = tango_periodic_source.get(key)) != null)
                  tangoPeriodic.removeTangoPeriodicListener(listener);
          }
      }
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