Code:
 var %lines = $calc($did(mp3,2).lines)


var %lines = $calc($did(dialog name,mp3list).lines)

Code:
 var %lines2 $did(mp3,2).sel


var %lines2 $did(dialog name,mp3list).sel

in your popup, you can do this .amsg # $+ %lines $+ / $+ %lines2