mail is high-priority

adda/concurrency/mail is high-priority:
. process`mailboxes can be used for impl'ing
the notifications system .
. notice that sched'priorities can change
when the mailbox is updated;
any mailbox could include commands that when run,
will change the mailed agent's priorities .
. so all mail reading should be high priority ?
. the mail could work like we have ours,
where the sender can know in some cases
that sent mail has a certain priority;
a service could declare that its priority
could never depend on its mail
due simply to the nature of the service .
. of course,
any of those self-declared priorities
are all relative to priorities known by the system:
eg, the highest priority is always
giving prompt service to the real-time user,
and that induces a high priority in all dependencies .