can be pulled out to the core types package

How about use Sub ? I find it use Close(Number(-1)) in the test function, but somehow it confused me at the first time.

Open() and Close() are paired

because it's "open position" and "close position"

I think the process for xmaker is Open -> Cover -> Close

Currently the flow will be the following

1. maker market receive BUY 1 TRADE -> Open(1)
2. hedge market place SELL 1 ORDER -> Cover(1)
3. hedge market receive SELL 1 TRADE -> Close(-1)

Both 2 and 3 are SELL side in the hedge market, but the delta are different (1 and -1). Somehow, it confuse me.