The ‘out of the box thinking moment’ here is clearly the trick that leads to the +/- z formulation. It’s smart and beautiful and less important if new or not. Einsteins uncle once taught him that Maths is the science where, if you don’t know something, you call it ‘x’ and you go on. :) The whole brilliance/innovation/breakthrough can lie in making the right/creative/smart assumptions. One needs uncommon insight called mathematical intuition to discover it by oneself, first or not. Simple and Beautiful! :)