[samibe]

I would like the option to add a value to a field for multiple songs (in batch edit mode) that might already have other values in it. 

For example, if I added a book of classical songs from various eras, filled in the genre field with the eras, and then later wanted to add "instrumental" to the genre field. I would have to filter the book by genre and then add "instrumental" to each group of songs. It would be nice if there was a checkbox to "Add" values to the field. Where values that are not shared by all selected songs could be grayed out and values that are shared by all can be added and/or removed.