Nexus and Dirac lines in topological materials

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.