code: public GroupSequenceType<ClassType<T>> getOrCreateGroupSequence()
     {
        Node node = childNode.getOrCreate("group-sequence");
        GroupSequenceType<ClassType<T>> groupSequence = new GroupSequenceTypeImpl<ClassType<T>>(this, "group-sequence", childNode, node);
        return groupSequence;
     }
docs: If not already created, a new <code>group-sequence</code> element with the given value will be created.
  Otherwise, the existing <code>group-sequence</code> element will be returned.
  @return  a new or existing instance of <code>GroupSequenceType<ClassType<T>></code>