Imagine you are running a stock exchange using the Promise Language specification

Initiating Promise:    100 IBM
Notice - accept: 5000 USD

Initiating Promise:    5000 USD
Notice - accept: 100 IBM


Note the structure is identical for both the Bid and the Ask.  Effectively this means everything is a commodity.

"Notice" messages become part of the contract.  Terms and late payments can be specified.  Clearing companies as well.