tup.getrelativedir() missing in tup lua api documentation

In the post at https://groups.google.com/forum/#!msg/tup-users/MKVeu9a3LHg/znEpZgCUTccJ you mention the lua function `tup.getrelativedir()`, but it is not included in the api documentation at http://gittup.org/tup/lua_parser.html. Is it part of the api, or could it be removed at some point?
