# What does the symbol ∟ mean?

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: