Chao-Kuo Lin's overlay/union/layer/stitch file system

Divergefs is a user level file server which layers arbitrary
number of directory trees on top of the base directory
mountpoint based on user specified rules from rulefile.  Its
uses include, but are not limited to, temporarily modifying
a few files on read-only media or speeding up compilation by
storing object files locally instead of on a distant file
server.