(future-cancel f)
Cancels the future, if possible.
user=> (def f (future (Thread/sleep 5000) (inc 0)))
#'user/f
user=> (future-cancel f)
true
user=> (future-cancelled? f)
true
user=> (def f (future (inc 0)))
#'user/f
user=> (future-cancel f)
false
user=> (future-cancelled? f)
false