OK,
I found this:

ToggleHiddenLine – to toggle hidden line display on or off quickly.
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script:moi.view.showHiddenLineCurves = !moi.view.showHiddenLineCurves; moi.view.showHiddenLineEdges = !moi.view.showHiddenLineEdges;
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But it doesn't work. I get the error shown here: