[Request] Would a %if(playlist)s tag be possible?

[HASJ]

It would be very handy when used in a config.txt file:
-o "D:%(uploader)s-%(upload_date)s-(%if(playlist)s_%(playlist_index)s) - %(title)s.%(ext)s"

Or even simplier, YDL could detect if the video is part of a playlist and automatically add which number the videos are in the playlist. Like:

-o "D:%(uploader)s-%(upload_date)s>-(%(playlist)s_%(playlist_index)s)< - %(title)s.%(ext)s"

< indicating that those file name tags should only be used if the tags are applicable to the video/URL. I'm guessing URL would be easier to do.