[Zope] _setOb vs. _setObject in BTreeFolder

What is the difference between _setObject and _setOb in a BTreeFolder? I discovered that if I use _setObject the BTreeFolder is not used, but when I use _setOb it used, but the documents don't appear in mybtreefolder/manage_main. BTreeFolder seems to have to storages: the inherited ones from Folder and the BTree storage. This is confusing. I think it would be better if BTreeFolder would delegate all methods to _tree, or rise an exception if you try to write the Folder storage. BTreeFolder doesn't display the addProdukt drop-down box properly, too. I use BTreeFolder version 0.2 from February 2001. Are others having this problems, too?