4.4 BSD includes a file system called ``nullfs'' that is identical to my Wrapfs. BSD's nullfs does not create any infrastructure for stacking; all it does is allow mounting one part of the file system in a different location. It proved useful as a template from which 4.4 BSD's Union file system was written [Pendry95]. The latter was developed by extending nullfs to merge the mount point file system and the mounted one, rather than blindly forward vnode and VFS operations to the new mount point.
The only contribution of 4.4 BSD to stacking is that it used an existing vnode interface in a manner similar to FiST. In fact, the way to write stackable file systems in 4.4 BSD is to take the template code for their nullfs, and adapt it to one's needs.