The rule is that financial operations do not lend themselves to innovation.

A Short History of Financial Euphoria by John Kenneth Galbraith.  Page 19.

As to new financial instruments, however, experience establishes a firm rule, and on few economic matters is understand-ing more important and frequently, indeed, more slight. The rule is that financial operations do not lend themselves to innovation. What is recurrently so described and celebrated is, without exception, a small variation on an established design, one that owes its distinctive character to the aforementioned brevity of the financial memory. The world of finance hails the invention of the wheel over and over again, often in a slightly more unstable version. All financial innovation involves, in one form or another, the creation of debt secured in greater or lesser adequacy by real assets. This was true in one of the earliest seeming marvels: when banks discovered that they could print bank notes and issue them to borrowers in a volume in excess of the hard-money deposits in the banks' strong rooms.  

 

The depositors could be counted upon, it was believed or hoped, not to come all at once for their money. There was no seeming limit to the debt that could thus be leveraged on a given volume of hard cash. A wonderful thing. The limit became apparent, however, when some alarming news, perhaps of the extent of the leverage itself, caused too many of the original depositors to want their money at the same time. All subsequent financial innovation has involved similar debt creation lever- aged against more limited assets with only modifications in the earlier design. All crises have involved debt that, in one fashion or an- other, has become dangerously out of scale in relation to the underlying means of payment.