Yes, it's called by one of the Java libraries. A `Callable` is basically a `Runnable`, except that it returns a result (in this case, a `Formula`).

I instantiate the `Callable` object [here](https://github.com/TeamAmalgam/kodkod/pull/31/files#L5R129), when I submit it to the `ExecutionCompletionService`. The ECS will then pass the `Callable` to a thread and invoke `call()`.