HUB: the central server that is the hub of the system

Stores:

• question data, including numbers of enquiries asking question and users answering it;
• other overall statistics, e.g. for the activity of different types of enquiry;
• some historical user answer records.

For answers, the hub will store some kind of cryptographic hash record of each user's answer history, in such a way that:

• any user can check that the returned record is correct;
• the records or hashes can be found quickly.

How to do this needs to be further thought through. If the records are changed, the hub knows that the local server records have changed, and that they must be discounted.