= Counter-Proposals and Offers =

When attempting to schedule an event, an attendee may suggest an alternative

time by issuing a counter-proposal in the form of a `COUNTER` message. Upon

receiving a counter-proposal, an organiser may then issue an updated

invitation with new details (potentially matching those suggested in the

counter-proposal, but not necessarily) for attendees to accept or decline.

Alternatively, an organiser may refuse to consider other times and send the

attendee concerned a `DECLINECOUNTER` message.

== Offers to Schedule ==

It can be useful for an attendee issuing such a counter-proposal to mark the

periods involved as tentatively occupied until they know that their proposal

has been accepted or not; it is also useful for the organiser to be able to

rely on such periods being temporarily reserved by attendees so that any

acceptance of a counter-proposal will succeed instead of failing because an

attendee has chosen to dedicate those periods to something else. Thus, an

attendee will make a temporary offer of scheduling for periods of time

associated with an event, expiring such an offer in case the organiser chooses

not to respond in a timely fashion (or at all).

== Managing Counter-Proposals ==

Each attendee invited to an event may respond with a counter-proposal. Thus,

an organiser needs to be able to collect multiple counter-proposals for each

event and to choose one to accept, if any are to be accepted at all. In

principle, an organiser could merge details of different proposals,

particularly if multiple periods are involved (and also if different attendees

are suggested in different proposals), with the resulting combination of

details being issued as a new request to all attendees.