VectorXYZT::mag() may be confusing since not four-vector magnitude
mag() is defined as
double mag() const {return std::hypot(x, y, z);}but the magnitude of a fourvector is traditionally defined as mass() (maybe up to a sign)
double mass() const {return sqrt(t*t - x*x - y*y - z*z);}This might lead to confusion when people start to use this in things like q2 = (ef-ei).mag().