<_description>
A 4D embedding of the real projective plane.
You can walk on the surface of the real projective plane or rotate it
in 4D or walk on it while it rotates in 4D. Inspired by Thomas
Banchoff's book "Beyond the Third Dimension: Geometry, Computer
Graphics, and Higher Dimensions", Scientific American Library, 1990.
https://en.wikipedia.org/wiki/Real_projective_plane
https://en.wikipedia.org/wiki/Roman_surface
https://en.wikipedia.org/wiki/Cross_cap
https://en.wikipedia.org/wiki/Moebius_strip
http://mathworld.wolfram.com/RealProjectivePlane.html
http://mathworld.wolfram.com/RomanSurface.html
http://mathworld.wolfram.com/Cross-Cap.html
http://mathworld.wolfram.com/MoebiusStrip.html
Written by Carsten Steger; 2014.