In relation to the following URL question, I would like to consider a proof for the one-sample case. https://stats.stackexchange.com/q/501493/401056
Definition
Consider the median of the average $$ \theta_{HL} = \mathrm{med}_{i \leq j} \left( \frac{X_i+X_j}{2}\right). $$ If the $X_j$ are I.I.D. according to a distribution $F(x-\theta)$ where $F$ has a density $f$ and is symmetric about $0$.
Theorem
$$ \sqrt n (\theta_{HL}-\theta) \overset{d}{\to} N\left(0, \frac{1}{12\left[\int f^2(x)dx \right]^2} \right) $$
Lehmann (1999) omits the proof. Can someone provide a proof or a hint?