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I'm trying to understand distributional RL, based on this article. In one of the equations, there is a symbol $\operatorname{sup dist}$.

\begin{align} \operatorname{sup dist}_{s, a} (R(s, a) + \gamma Z(s', a^*), Z(s, a)) \\ s' \sim p(\cdot \mid s, a) \end{align}

What does $\operatorname{sup dist}$ mean?