Run time NGDP betting with a value added tax

We are going to three color the central bank trades, triple entry accounting.  We have variable savings, loans and taxes. 

Basic contract:
Traders trade in the monopoly tax dollar, they are guaranteed that taxes are only collected in dollars.  In return for the monopoly, the pit boss charges a variable value added tax on transaction amount. 

The process goes as:
loans are balanced against savings and interest payments made, in and out, when NGDP is out of variance.  In additions, some varying amount of that flow is retained if government interest charges  are out of balance. Government interest charges have to be limited to 3% of the economy.

If government borrows excessively, interest charges go up and possibly government gets some of that back via the tax, like seignorage. Keep the value added tax ratio set so as eliminate an arbitrage.