"The overall theme is a very popular one - I'd be astonished if you were unaware of it unless you lived on Mars. "
I leave in the US and was unaware of it. But Google was aware...
The line in the preamble about mirror symmetry is somewhat misleading. If you trace the object in the natural way with lines you do not get symmetry. HIghlighting the cells gives symmetry; you get symmetry if you draw curves as long as you don't mind re-entering already used cells.