Perhaps it's too late at night, but I can't think of a nice way to do this.
I've started a bunch of asynchronous downloads, and I want to wait until they all complete before the program terminates. This leads me to believe I should increment something when a download starts, and decrement it when it finishes. But then how do I wait until the count is 0 again?
Semaphores sort of work in the opposite way in that you block when there are no resources available, not when they're all available (blocks when count is 0, rather than non-zero).
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