+ If the optional argument is supplied, it must be of the form "p/foo"
+ or "m/bar" (which refer to the playlist named "foo" and the mood named
+ "bar", respectively). The command then restricts its output to the set
+ of files which are admissible with respect to the thusly identified
+ mood or playlist.
+
+ If no argument is given, or if the argument is the special value ".",
+ the current mood or playlist is assumed.