use std use "sem" pkg thread =	type future(@a) = struct		sem	: sem		val	: @a	;; 	generic mkfut	: (-> future(@a)#)	generic futput	: (fut : future(@a)#, val : @a -> void)	generic futget	: (fut : future(@a)# -> @a);; generic mkfut = {	-> std.mk([.sem = mksem(0)])} generic futput = {fut, val	fut.val = val	sempost(&fut.sem)} generic futget = {fut	var val		semwait(&fut.sem)	val = fut.val	std.free(fut)	-> val}