Let x+ and x- be the light cone coordinates x + t and x - t. The coordinate x- = log x+. The equations dx- = dx-/x+ and x+ = x+ dx- are like ds = dQ/T and dE = p dV. d- = d- dx+ D- = d- + d- dx+ In mechanics we have L = T - V, where L = L dt. This is a complex differential form just like x+ = x+ dx- and d- = d- dx+. Has anyone seen D- = d- + d- dx+?