docs: protected void addNestedFormatter(String elementName, CmsXmlContentDefinition contentDefinition)
      throws CmsXmlException {
  
          if (contentDefinition.getSchemaType(elementName) == null) {
              throw new CmsXmlException(
                  Messages.get().container(Messages.ERR_XMLCONTENT_INVALID_ELEM_MAPPING_1, elementName));
          }
          m_nestedFormatterElements.add(elementName);
      }