Constructing Metric from Null rays (up to a Conformal factor)

Null rays are the zeros of the length functional, albeit cumbersome `zero's to work with.

Here we need a generalization of Weierstrass factorization theorem to enable us to construct a functional from its zeros (and naturally a generalization of Mittag-Leffler)

$$ds^2=(cdt-dx)(cdt+dx)$$