Lorentzian angles and trigonometry including lightlike vectors

Understand

We define a concept of Lorentzian angle that works even when one or both of the directions involved is null (lightlike).

• Such angles play a role in Regge-Calculus, in the boundary- and corner- terms for the gravitational action, and in the Lorentzian Gauss-Bonnet theorem (for which we provide a proof).