Must be a way to subtract...
On my old version of Gimp you could draw a selection with the lasso tool, copy>paste and use the eraser to trim unwanted matter before anchoring the selection where needed.
I have tried all of a variety of how-to tips with only more confusion to show for it.
This I discovered on my own: you can make a selection, copy it to the clipboard, create a new file from the matter on the clipboard.....and, by God, you can erase from that! Woo!
But this is not as handy, especially if you want to be especially intricate with the revised matter you want to paste.
Using the control key and drawing the area you want to eliminate does not seem to work. Surely I'm doing something wrong but this must be really tricky.
Brothers and sisters, an you help ???? /Beriganesque