Allow the use of existing masks when adding or subtracting masks

Add and subtracting masks is very useful in refining your mask. Being able to add or subtract and existing mask would be (IMO) a great improvement in this feature.

Many times I will labour over tweaking the mask for my subject, then I want to subtract that mast from my background so background adjustments don’t affect my subject. The background/landscape/sky masks will often include areas of the subject which interfere with processing. 

Here’s an example.
Selecting “Subject” results in tree branches being included and misses many detailed tail feathers.

Selecting (in this case) “Vegetation” includes those same tail features as part of the selection. 

I can spend a great deal of time painstakingly finessing my subject selection and would like to subtract that from the background.

I did find this as a feature request in the Lightroom area so I hope this is not out of bounds to add to the ACR area. ACR seems to have new features tested out before being added elsewhere (and ACR is my tool of choice over Lightroom), so I’m hoping to see this in ACR.