I encountered this symbol $\lefthalfcup$ ∟ in the statement of Besicovitch derivation theorem. It says that the Radon-Nykodym decomposition of the given measure $\nu$ is $\nu=f\mu+\nu^s$, $\nu^s$ is singular with respect to $\mu$ and
$$ \nu^s∟ E $$
where $E$ is a specific set.
I don't understand what that symbol means. It also appears in Federer-Vol'pert theorem and it may have something to do with distributional derivatives as seen here, but I really can't figure it out.
Any help is appreciated, thank you.
Screenshot of page 24 of The Comprehensive $\LaTeX$ symbol list: