Erase mask?

Bought SNS a while ago, but just started using on a regular basis for a large amount of images I brought back from a recent trip. Great output, hardly requires any tweaking.
Anyway, I was making use of the mask functionality, painting on the mask with one of the brushes. Then I found myself in need of erasing part of the mask that I had drawn, but couldn't find a way to do that. I can get rid of the entire mask, but there doesn't seem to be an erase brush to prune back part of an existing mask. Would like to request that as a feature in a future update, if it's indeed not available - or did I overlook something?
Thanks.
Anyway, I was making use of the mask functionality, painting on the mask with one of the brushes. Then I found myself in need of erasing part of the mask that I had drawn, but couldn't find a way to do that. I can get rid of the entire mask, but there doesn't seem to be an erase brush to prune back part of an existing mask. Would like to request that as a feature in a future update, if it's indeed not available - or did I overlook something?
Thanks.