Use DPC with non-trivial predicate

[sam-steffen]

How can I use DPC to instantiate the very simple "conserve" predicate required to encode the basic user-defined asset from the reference paper [1]?

I could not figure out how to do this. The current implementation hardcodes an "always accept" predicate and it's unclear how to instantiate the "conserve" predicate: the sum of inputs should equal the sum of outputs.

[1] https://eprint.iacr.org/2018/962

[sam-steffen]

My current plan seems much more complicated than expected; am I on the right track?

Step 1: Create R1CS
It seems like I'd have to implement the ConstraintSynthesizer trait generating a R1CS for the predicate. But, how can I represent the "conserve" predicate as a R1CS? I know there are reductions from arithmetic circuits to R1CS [2] but don't know a standalone tool which automates this (I could not figure out how to do this in libsnark).

Step 2: Integrate predicate
The "always accept" predicate EmptyPredicateCircuit is hard-coded in the plain_dpc module (even outside the tests). Hence, I'd have to refactor the code to allow a different predicate.

[2] https://eprint.iacr.org/2013/507.pdf