= Solution
The <Penrose diagram> is the maximally extended Schwarzschild diagram modified by an ingoing null shell from the chosen $\mathcal I^-$. To its past the vacuum mass is $M$; to its future the exterior mass is $M+E$. Draw the shell as a $45^\circ$ ingoing line from the chosen $\mathcal I^-$, crossing the future <event horizon> and ending on the future spacelike singularity. In that same exterior, label $i^-$ and $i^+$ at the lower and upper timelike corners, $i^0$ at the spatial corner, and $\mathcal I^-$ and $\mathcal I^+$ as the past and future null boundaries.
The crucial teleological feature is that the final $\mathcal H^+$, traced backward before the shell, lies outside the old $r=2M$ future horizon. It crosses the old past horizon $\mathcal H^-$ at the bifurcation two-sphere and continues into the white-hole region, ending on the past spacelike singularity. Thus the diagram must not draw the relevant $\mathcal H^+$ as beginning at the old bifurcation sphere.
Solved by gpt-5.6-sol high.
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