More explanations on Monero's implementation of bulletproofs could be found on youtube fondajo channel in a [conversation with Sarang Noether](https://www.youtube.com/watch?v=6lEWqIMLzUU).
To accomplish this, two kind of ring signatures were constructed: One ring signature for the whole transaction (to prove the sum is 0), and a set of ring signatures for the subsets of transaction bits (to prove the outputs are positive numbers), then combined together using originally Schnorr signatures (and later replaced by Borromean ring signature).
