To those offering up suggestions of "seticoin" and similar ideas, the reason why this hasn't been done is because its prohibitively difficult to build a PoW sys

To those offering up suggestions of "seticoin" and similar ideas, the reason why this hasn't been done is because its prohibitively difficult to build a PoW system using useful work because useful work usually cannot be verified as correct without redoing all of the work. Another problem is that the inherent nature of useful work generally precludes it from being delivered in units of controllable and predictable difficulty.