/vm/src/circuit/r1cs.rs: fn store_reg : build constraints

[hjlpb]

[yoichi-nexus]

I think there's something to fix here. The original version doesn't mention x1 x2 through x32 variables without 's. And the proposed fix makes sense to me.

[yoichi-nexus]

@hjlpb Do you want to file a pull-request for this change?

[sjudson]

Closing w/ merged PR to fix: #205

[taint160596]

Thank you