Coxeter p.26 no.2
Points P,Q,R,S are draggable (they represent the original quadrangle). A, B, C are the diagonal points A1,..., C2 are created according to the problem given in the book. The notation, for example, "A1B2C2" represents a line, and that these 3 points are collinear (which we may or may not have to prove...I'm not sure yet). Note that the "new" quadrilateral (orangle, yellow, pink, silver) is supposed to have the same diagonal triangle as the original quadrangle. I think we have to prove that it does.