This library provides:

* an Atomic class that guarantees atomic updates to its contained value

The Atomic class provides accessors for the contained "value" plus two
update methods:

* update will run the provided block, passing the current value and
replacing it with the block result iff the value has not been changed
in the mean time. It may run the block repeatedly if there are other
concurrent updates in progress.
* try_update will run the provided block, passing the current value and
replacing it with the block result. If the value changes before the
update can happen, it will throw Atomic::ConcurrentUpdateError.