Is there some permissions setup that would allow a user, say,
john, to add a file to a directory
d, but not be able to remove an existing file owned by another user, say,
My understanding is that this is not possible, since one needs execute permissions to add a file to the directory, but this also gives on the power to unlink any file in the directory.
(I'm using Mac OS 10.9, but this question presumably applies to all POSIX-ish systems.)