Quoting Liam R E Quin <[EMAIL PROTECTED]>:
> It's not entirely true to say you had to have used paths from the start,
> since you can save the first selection to a channel, make the new one
> with paths, including an approximate, much larger outer rectangle,
> and then convert to selection and intersect with the saved selection.
GIMP 2.4 (RC2) allows you to add/subtract/intersect a path
to/from/with the selection by using the SHIFT/CTRL/(SHIFT+CTRL) keys
when you hit ENTER (or click on the "Create Selection from Path"
button in the Tool Options).
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