 Monotonicity of boundary derivatives of mapping-out functions

For nested compact H-hulls whose closures avoid $x\in \mathbb{R}$, the boundary derivative is an excursion avoidance probability. Avoiding the larger hull is a subset of avoiding the smaller one, which proves the derivative inequality. For a hull contained in $\{|z|\le R\}$ and $|x|>R$, comparison with the filled half-disc gives the sharp bound $1-R^2/x^2\le g_A^{\prime }(x)\le 1$, since that half-disc has mapping-out function $z+R^2/z$. The empty hull and filled half-disc attain the two bounds.