Digital cash issuess...

As a consequence, our cash system is much more efficient in both computation and communication complexity than previously proposed systems."

I take this with a grain of salt; see below.

In our opinion, the most interesting of these is that the entire cash system (including all the extensions) can be incorporated straightforwardly in a setting based on wallets with observers,

I am not surprised that they find this interesting; David Chaum has patented all the observer protocols. Having read these protocols in the original, I can say this is not much of an advantage. The observer protocols are tremendously expensive computationally. Anything you build on top of it won't be any faster. Eric