code: public Enumeration<String> getElements() {
          AttributeNameEnumeration elements = new AttributeNameEnumeration();
          elements.addElement(VERSION);
          elements.addElement(SERIAL_NUMBER);
          elements.addElement(ALGORITHM_ID);
          elements.addElement(ISSUER);
          elements.addElement(VALIDITY);
          elements.addElement(SUBJECT);
          elements.addElement(KEY);
          elements.addElement(ISSUER_ID);
          elements.addElement(SUBJECT_ID);
          elements.addElement(EXTENSIONS);
  
          return elements.elements();
      }
docs: Return an enumeration of names of attributes existing within this
  attribute.