_.ThreadCooperative threads.
val async : (unit -> 'a) -> 'a tasync f creates a new thread of control which executes f () and returns the corresponding thread handle. The thread terminates whenever f () returns a value or raises an exception.
val await : 'a t -> ('a, [ `Async_exn of exn ]) Stdlib.resultawait t blocks on the termination of t.
val return : 'a -> 'a treturn () is a pre-terminated thread handle.