Years ago I also did some experimentation w/ midi-device and SRS ( https://www.youtube.com/watch?v=a6tvHMvF8Mo ), where the focus was on ear-training rather than score-learning.

Clef seems to be a pretty strong attempt at a high difficulty UX. I've created an account and will be giving it a go. Wishing you luck, and thanks for sharing.

What a great application of spaced-repetition! How do you determine _when_ to prompt again?