Lambda the Ultimate USENIX2003: "We would like to "cd /lib" and descend into the "directory" that contains only library files, grouped into subdirectories "root", "usr", "local". We would like to "cd /bin|/sbin" and get to the root of the "filesystem tree" that contains only executable files (again, grouped into "subdirectories"). We may even do "cd !src" to get the view of the filesystem without any source files. The Logical File System described in this talk (LISFS for short, not to be confused with the Log File system) can accomplish all that -- now. The prototype implementation is fully functional and reasonably fast. An ls-benchmark shows hardly any performance overhead of far more expressive filesystem queries."

He's referring to Logical Information Systems.