  We consider the $Z_2$ topology of the Dirac lines, i.e., lines of band
contacts, on an example of graphite. Four lines --- three with topological
charge $N_1=1$ each and one with $N_1=-1$ --- merge together near the H-point
and annihilate due to summation law $1+1+1-1=0$. The merging point is similar
to the real-space nexus, an analog of the Dirac monopole at which the $Z_2$
strings terminate.
