Thank you.
I explain my intend of doing it. I made a macro that is used for chamfering a
cylinder and it is necessary to take the torus's inner surface in some
conditions. So I intersect sphere and torus with negative major_radius and
sometimes found these black lines.

Maybe the same demand can be done by using polynomial's torus and I would like
to try some day. Because I thought the black line may be produced by the
bounding cylinder of the torus object that the document describes.

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