create n create a semaphore with initial value n (a positive integer). Raise Invalid_argument if n <= 0

acquire k s block until the semaphore value is >= k (a positive integer), then atomically decrement the semaphore value by k. Raise Invalid_argument if k <= 0

release k s atomically increment the semaphore value by k (a positive integer). Raise Invalid_argument if k <= 0

execute_with_weight s k f acquire the semaphore with k, then run f (), and finally release the semaphore with the same value k (even in case of failure in the execution of f). Return the value of f () or re-raise the exception if any.

execute s f same as {execute_with_weight} s 1 f