I want to be able to hold onto a promise such that I can create new children, without blocking cancellation propagation from any existing children. With such a mechanism, I should have a callback that's invoked as soon as all children have requested cancel, so I can throw away the parent promise I'm holding.

The idea here is so I can dedupe requests for an upstream asynchronous resource (such as loading images from the network), and allow for cancelling the upstream asynchronous resource as soon as all children have declared they're no longer interested in it.