replacing subset of image

[herman]

I messed up on a star halo I was doctoring using StarHaloReducer.  I'd like to replace the subset of the image where the star is to an earlier version I have.  Is there a way with PixelMath, for example, to just perform an operation on a subset of the image?

[herman]

I just found the "Substitute with Preview" script that seems to do the trick.