I wrote a very niche App for people who like Opera. It's most important purpose is to display Opera track listing meta-data correctly. (You can see comparison screen shots at https://ariascribe.com). Along the way, I discovered that, even though the genre "opera" has long been recognized, and even has an official ID3v1 tag (103), I have never ever seen the tag used in CDs or downloads. My App has to figure out whether or not an album in a users collection is an Opera. Should have been trivial. Was in fact hard. Very hard.