I was using that earlier.
Code:
    write -w"$+($msn,*)" Mp3Rank.txt $msn : $calc($gettok($read(Mp3Rank.txt,n,$msn),2,58) + 1)


Doesn't work for me. Instead of replacing the string, it just appends to the end of it.

$msn = current song playing, in the format "Artist - Song : [num]"