  We study under which general conditions a pair of Dirac points in the
electronic spectrum of a two-dimensional crystal may merge into a single one.
The merging signals a topological transition between a semi-metallic phase and
a band insulator. We derive a universal Hamiltonian that describes the physical
properties of the transition, which is controlled by a single parameter, and
analyze the Landau-level spectrum in its vicinity. This merging may be observed
in the organic salt alpha-(BEDT-TTF)_2 I_3 or in an optical lattice of cold
atoms simulating deformed graphene.
