code: Double getSum(String multiRelationName, EntityFilter filter,
              String numericAttributeName) {
          return instance.getSum(this, multiRelationName, filter,
                  numericAttributeName);
      }
docs: Get a total of an attribute thru a multi-relation possibly slicing by a
  filter.
  
  @param multiRelationName
  @param filter
  @param numericAttributeName
  @return total of an attribute.