tennis takes an hour or two; a member’s subscription is valid for a season or
for a year, or even longer. This shows there are at least two different sub
subscriptions is a dynamic domain: members join and leave, pay or don’t pay
and its lights: treat membership as static
lights is a third subproblem because it straddles two tempi. Items to be
billed are games of tennis, recorded in the fast problem. Billing and payment
proceeds at a slower pace, perhaps monthly. Late payers and defaulters may
find their membership suspended.