openscad screw threads with linear_extrude

It's always bugged me that you can't make screw threads easily with openscad's linear_extrude - and then I realized, on my bike ride home from work, that actually you can!
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The main problem is that people usually think of a screw thread in terms of a *vertical* cross section, but openscad needs a *horizontal* cross-section for linear extrusion. But this isn't a fundamental limitation, it's a failure of imagination! So imagine cutting a screw in half with a hacksaw. Look at the shape the