code: public DeviceDataHistory[] attribute_history(final DeviceProxy deviceProxy, final String attname)
              throws DevFailed {
          int hist_depth = 10;
          final DbDatum data = get_property(deviceProxy, "poll_ring_depth");
          if (!data.is_empty()) {
              hist_depth = data.extractLong();
          }
          return attribute_history(deviceProxy, attname, hist_depth);
      }
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