Transparency using brushes

[Marco D]

I wrote this last night:


I hadn't though of using an Opacity Mask and I came up with Marco's approach.
I stopped at that point as once you Subtract, you are stuck with a fixed shape.

Thank you both @PhilM & @handrawn as your approach retains the means to edit and experiment.
Another arrow for my quiver.

Acorn

My suggestion was just to indicate another possible approach to the problem. The solution through the Opacity Mask is certainly the preferred one because it does not modify the original image and makes everything more easily editable, as Acorn also mentioned.