[Zope] Is this a feature or a bug ?

Hello, I am using Zope 2.3.3. I have coded a python class inheriting from ObjectManager (and others, as explained in the ZDG). Is this a feature or a Zope bug that instances of that class _cannot_ contain other instances of the _same_ class, just like a Zope Folder obect can contain other Folder objects? I have also tried by inheriting from the Folder class instead of ObjectManager. It does not solve the problem. Note the objects are created but not where expected, instead they are created in the first object in the url being a Folder object. This is a strange behavior, isn't it? Thanks in advance for your answers, Philippe Bocquillon