frequency ~ <num>
channels ~ <num>
num_played ~ <num>
+ image_id ~ <num>
+ lyrics_id ~ <num>
Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
<num>. Matches an audio file iff the condition <val> ~ <num> is
satisfied where val is the corresponding value of the audio file
-(value of the year tag, bitrate in kbit/s, frequency in Hz, channel
-count, play count).
+(value of the year tag, bitrate in kbit/s, etc.).
The year tag is special as its value is undefined if the audio file
has no year tag or the content of the year tag is not a number. Such