docs: public String firstMandatoryNullProperty()
      {
          for (String property : model.propertyList)
          {
              if (
                      model.mandatorySet.contains(property) && 
                      model.defaultMap.get(property) == null &&
                  data.get(property) == null
                      )
              {
                  return property;
              }
          }
          return firstMandatoryNullProperty(model.propertyList);
      }