In Search of Genuine DigiCash

It's the behavior of the financial instrument I'm talking about. At some point, the principal goes away and has to be called from wherever it is (a bank account, the money market, etc.) to meet a cashed-out piece of digicash. In the meantime it earns interest. Thus it has principal, and interest, and it is called. It's a callable bond. Now, consider a promissory note which is redeemable on demand and which pays interest at redemption. This instrument has the same financial properties as a callable bond. Pop Quiz: why is this promissory note _not_ actually a callable bond? Eric