  It is well known that a single Dirac cone at high-symmetry point (HSP) of a
Brillouin zone, akin to the one in graphenes' band structure, can not appear as
the only quasiparticle at the Fermi level in two-dimensional (2D), non-magnetic
materials. Here we found two layer groups with time-reversal symmetry, among
all possible both without- and with spin-orbit coupling, that host one Dirac
cone at HSP and we show which additional dispersions appear: a pair of Dirac
lines on opposite BZ edges and a pair of Dirac cones that can be moved but not
removed by symmetry preserving perturbations, on the other two BZ edges. We
illustrate our theory by a tight-binding band structure and discuss real 2D
materials that belong to one of the two symmetry groups. Finally, we single out
inconsistencies in the literature showing that it is better to focus scientist
attention to a research topics itself, instead to group of authors only (e.g.
from one university, country etc.) dealing with the same topics. On the other
hand, repetitions of other scientist results without due citation, destroys the
spirit of science itself.
