In message , at 
17:42:58 on Sat, 7 Jun 2008, Mark Goodge 
 remarked:
>In a compulsory ticket area, everyone requires a ticket. In a
>non-compulsory ticket area, only travellers require tickets.

I agree with most of what you say, but in a non-compulsory ticket area 
travellers are sometimes excused tickets too - if they haven't yet 
encountered an opportunity to buy one. Once they have encountered that 
opportunity, then they *are* required to both have, and make available 
for inspection, that ticket.
-- 
Roland Perry