a$. The coordinate $\lambda_0$ takes value $\lambda_0=-b$ at the intersections with $\mathsf{x}$-axis and value $\lambda_0=0$ at the reflection points off the arcs of ${\pazocal{E}}$ where the restricted metric is time-like. On the other hand, $\lambda_0$ takes value $\lambda_0=\gamma$ at the touching points with the caustic, and $\lambda_0=0$ at the reflection points off the arcs of ${\pazocal{E}}$ where the restricte