Return t if file FILENAME names a directory you can open.
For the value to be t, FILENAME must specify the name of a directory as a file,
and the directory must allow you to open files in it. In order to use a
directory as a buffer's current directory, this predicate must return true.
A directory name spec may be given instead; then the value is t
if the directory so specified exists and really is a readable and
searchable directory. (fn FILENAME)