which implement the actual CAPTCHA requirements. Note that bob specifies the
 URLs of the ticket servers: this is abstracted out to let the challenge
 change over time, as CAPTCHA technology changes. The TS is providing a
 *service*, that of enforcing the "my time for your time" rule. Each server
 publishes what kinds of tests they do, and Bob picks whichever ones he wants
 to impose upon new senders.

 The result of the CAPTCHA challenge is a signed Ticket that gets put in the
 message.