[math-fun] antiprism oddity

Twist "top" face half a "click" wrt to its opponent. This makes an ordinary prism with rectangular faces diagonally slashed. If you twist a whole click the other way, the whole figure will mirror-image. I.e., the slashes flip to the opposite diagonal. You could make a prank animation of the top face continuously rotating wrt the bottom, by repeatedly fading out the slash and fading up its mirror image, or have both slashes present at the "prism points". --rwg