I submit a proposal, titled "Promises do an acceptable job of handling this,
but a specialized mechanism would probably be more convenient and less
error-prone":
Create a rule, titled "Trade Offers":
A person (the Seller) CAN create a Trade Offer by announcement,
specifying a set of assets e offers to trade (the Wares), and a
set of assets e wishes to receive in exchange (the Tender).
Any other person (the Buyer) CAN accept a Trade Offer by
announcement. When this happens, if the Seller owns the Wares
and CAN transfer them to the Buyer, and the Buyer owns the
Tender and CAN transfer it to the Seller, then the Wares are
transferred to the Buyer, the Tender is transferred to the
Seller, and the Trade Offer is destroyed. If any part of this
transaction would fail, the entire transaction fails.
A Seller can destroy (syn. revoke) eir own Trade Offers by
announcement. Trade Offers are automatically destroyed 4 days
after they are created.
—Machiavelli
