Pythagoras, Pythagorean, Euclidean, Gravitational Lensing, Symmetry.
the two red lines. You will end up again in a final state indistinguishable from the initial one. The swapped angles show this time: α = γ and β = δ. Finally, take again the last system and add a blue straight line crossing perpendicularly one of the intersections of red-green lines. Now it is