= Solution
Assume the avoidance formula. Given another admissible hull $C$, put $D=A\cup\psi_A^{-1}(C)$ with the bounded filling. Uniqueness of the normalized maps gives
$$
\psi_D=\psi_C\circ\psi_A,
\qquad
\psi_D'(0)=\psi_C'(0)\psi_A'(0).
$$
Therefore
$$
\begin{aligned}
\mathbb P(\psi_A(\gamma)\cap C=\varnothing\mid\gamma\cap A=\varnothing)
&=\frac{\mathbb P(\gamma\cap D=\varnothing)}
{\mathbb P(\gamma\cap A=\varnothing)}\\
&=\frac{\psi_D'(0)^\alpha}{\psi_A'(0)^\alpha}
=\psi_C'(0)^\alpha.
\end{aligned}
$$
These avoidance events determine the law of a simple closed random set. They agree with those of $\gamma$, so the conditional mapped law equals the original law. Hence the avoidance formula implies the <chordal restriction property>.
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