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There is an 'undo' button at the top-left. Though sometimes it didn't register touches on my iPad.

ah, sure enough. I limit my suggestion to an eraser then.

Their Web Shape demo (http://webdemo.visionobjects.com/shape.html?locale=default#) lets you "scratch out" objects - perhaps they could implement that here? It's what I usually do when writing equations quickly, anyway.