Yes, there is. Press and hold SHIFT.
To erase - select object, click Erase Tool, press and hold left mouse button, 'paint' over the part you want to erase.
To unerase - select object, click Erase Tool, press and hold left mouse button, PRESS AND HOLD SHIFT KEY, 'paint' over the part you want to unerase.
UN-erase, De-erase, Anti-erase, Reverse-erase? Non of them seem like real words
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