Exercise 4, § 3

If G is a graph, show that $\emptyset \circ \mathrm{G} = \mathrm{G} \circ \emptyset = \emptyset$, and that $\overset{-1}{\mathrm{G}} \circ \mathrm{G} = \emptyset$ if and only if $\mathrm{G} = \emptyset$.