code: public GroupsType<ConstraintType<T>> getOrCreateGroups()
     {
        Node node = childNode.getOrCreate("groups");
        GroupsType<ConstraintType<T>> groups = new GroupsTypeImpl<ConstraintType<T>>(this, "groups", childNode, node);
        return groups;
     }
docs: If not already created, a new <code>groups</code> element with the given value will be created.
  Otherwise, the existing <code>groups</code> element will be returned.
  @return  a new or existing instance of <code>GroupsType<ConstraintType<T>></code>