Meaning of PROBE-FILE for direction output

[Glenn S. Burke <GSB@MIT-MC>]

Another timing problem with this is that any way of doing it which
involves actually doing the open might leave an entry in the
filesystem, even though only temporarily.  I would not want to have to
simulate any hairy (tops-20, vms, especially version 4) operating
system's algorithm for determining if i had access -- ergo, i would
implement it by doing the open to see if it succeeds.