The examples that I have read on this are about a single asset getting transferred from one peer to another. But in most asset transactions it's a bidirectional transaction - cash on one-leg and the asset on the other leg. How does one accomplish this on the block-chain? I was trying to relate this to how a bitcoin exchange works (BTC to USD and vice versa) - which I presume is a solved problem. Is there any white-paper explaining how this works? Does Multchain support this today?