Consider a ledger swap

 he human ants to take cash from the S/L and send it o a trading pit. The S/L pit doesn't care, the market size will be refilled. The cash is in that hands of your designated trading bot, and reset once to the pit, under you account and signature. The S/L does not track it, in fact, the entire system is designed to rebid the past so it may be erased.

So, my trading bot is a bit more complex than a contract, but is still made up of small contracts. If a trusted miner wants to run ethereum as an automated investment manager, fine, take the bearer cash; these cards will recognize the cash if they know the ledger contracts. So, all currency systems need ledger and cash forms, otherwise they get left out of the game. Pure cash create those simple stable points on the contract graph, allow us to make complex from assemblies.  We processors can switch easier and sooner with cash transactions. Their timeout queue, managed in the kernel, will not congest with more use of pure digital cash.  Each card, and processor obeys the no double spending. If ledger queues get long, we get fails to deliver (recall before timeout). Fails first cause insurance fees and payments. After that, call the humans.

Pure bearer digital cash

Each card can verify secure digits, and interpret cash if it has the ledger protocol. So i defers a call to a trusted miner, and once acquired can be kept in the users possession in a spectre standard instruction cache. Any contract requiring swaps between the ledgers should always extract the cash first.  It is a bit more risky but a lot more smooth and you will not be charging your card batteries so much. Cash makes on line transactions go better, again, shortens the calls to trusted miner, these are special maps of the instruction that must be kept in kernel space. Always carry lots of cash in your smart card.  

The rules for cash is one, has the standard currency identifier and correspondent trusted miner (or banks or accountant or..) Second, has the privately exposed cash signature for the ledger system in question.  When any cad downloads the ledger protocol is also down load the cash protocols, all simple contracts.  So exchanging cash always work when both parties agree to the exchange currency. But the cards, like any wallet, carries multiple currencies of various sort without ledger protocol, and these can be passed.