If there is more than one effect of an event e, we write
for the time of the main effect, and 
for the time of the effect of changing f. For instance, flying from
Glasgow to London has as its resulting situation the situation where
you arrive in London. However, another effect of this event is to no
longer have your ticket, as the air-hostess takes it from you. The
situation where she takes the ticket off you is picked out by
. At this
situation, the fluent Has(ticket) is also made false. We use other
situations like the time the airline-steward takes your ticket, rather
than explicit times, as explicit times, like all numerical values are
less natural--the numbers are hard to get. The statement that you no
longer have your ticket after the air-hostess takes it is very
intuitive, while the statement that your no longer have your ticket
after n minutes, for some definition of n is not.