Thank you for this interesting discussion. It is my sense that repetition at the "just right" or even "slightly too easy" level can indeed be very beneficial. My DD "got" borrowing across zeros immediately, but she has benefited from practice, which has given her great automaticity. However, when she is asked to do review math on the single-digit level she rebels. I don't think this is HARMING her mastery of those facts, but it isn't exciting her about math, that's for sure.