Presence service identifiers

I just realized that one can save a lot of hassle by requiring that a
presentity's identifier can be used to derive the presence service
identifier of the service that provides subscriptions to the
presentity.  I suggest we follow the SIMP rule.  A presence URI that
specifies a presentity may not have a local part of 'notifier', and
the presence service identifier associated with a presentity is
derived from it by replacing the local part with the string
'notifier'.  A presence URI specifies a presence service only if its
local part is the string 'notifier'.

John



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