Cancelable promises proposal

[Boris Zbarsky]

On 8/2/15 8:43 PM, Glen Huang wrote:
> You can think it as that each promise keeps a list of its child promises, when the same callback is passed to .ignore() it sets a flag on the corresponding child promise so that when itself resolves/rejects, it won't pass that state to that child promise

There seems to be a potential footgun here.  Say I call then() twice 
with the same function.  Now I have two child promises.  If I now call 
ignore() with that function, which of the child promises is ignored? 
Both?  Just one of them?  Something else?

This situation doesn't _quite_ arise with add/removeEventListener, 
because you can't addEventListener a given listener twice to the same 
event target.  So a removeEventListener can see at most one matching 
listener.

-Boris