Devise scheme for policing document reuse

Certain documents might be considered to be owned under the umbrella of some external blockchain token. E.g. the minted value of token 1 may be derived from documents A & B; therefore token 2 cannot also be derived from A & B, but could instead be derived from C & D.

Likely this will have to be policed outside of the happ, because it is difficult to credibly maintain a list of nullifiers in the DHT. However, checking all document IDs on-chain is likely to by gas hungry if many documents are being aggregated.