Remark: Corrected chart coverage

The two $5a$ charts cover compact past-null non-tip regions after a finite partition: at least one of $|\gamma |$ and $|\upsilon |$ is bounded below.

For $5z$, the formulas above cover the non-tip region within the additional domain $\beta -\gamma \ge c_0>0$. Separation of $b-z$ alone does not imply that inequality: a nonzero future-null $b-z$ can be parallel to $k$. Outside this domain one must use another chart or the independent cone-radius formula below. No global coverage is claimed from these two denominators alone.