code: public String code(String classificationName, String elementName) {
          assertArgumentNotNull("classificationName", classificationName);
          assertArgumentNotNull("elementName", elementName);
          final ClassificationMeta meta = findClassificationMeta((String) classificationName, () -> {
              return "code('" + classificationName + "', '" + elementName + "')";
          });
          final Classification cls = meta.nameOf(elementName);
          assertClassificationByNameExists(classificationName, elementName, cls);
          return cls.code();
      }
docs: Get classification code.
  @param classificationName The name of classification. (NotNull)
  @param elementName The name of classification element. (NotNull)
  @return The found code of classification. (NotNull: if not found, throws exception)