At the risk of perpetuating a postmortem equestrian flogging, I have to strongly echo the request for hiding cautionaries.
Yes, I do understand that flows are able to accomplish this. But typesetting a large collection of exercises (such as Melodia, for example), requires an approach to flow management that feels supremely cumbersome. Melodia has several hundred exercises per chapter. It truly is best conceived as one simple, continuous document, one instrument, one layout. 300+ flows is a daunting prospect, when all that I need to do is hide a simple cautionary key/clef/meter.
I don’t pretend to understand the programming implications involved though.