code: public void reverse(AverageData data,
                          Object value) {
          if (value != null) {
              data.count--;
              data.total = data.total.subtract( (BigDecimal) value );
          }
      }
docs: /* (non-Javadoc)
  @see org.kie.base.accumulators.AccumulateFunction#reverse(java.lang.Object, java.lang.Object)