(sleep-until TIME)Suspend reading commands from this stream until TIME (in seconds).
Commands already read will still be executed; sleep-until inside
progn won’t delay execution of the rest of the progn’s contents.
Time is measured according to this stream’s clock, as set by
set-clock; if never set, the first sleep-until sets it to 0
(so initially (sleep-until TIME) is the same as (sleep-for TIME)).
Returns the number of seconds until TIME.