  Consider a Dirac operator defined on the whole plane with a mass term of size
m supported outside a domain Omega. We give a simple proof for the norm
resolvent convergence, as m goes to infinity, of this operator to a Dirac
operator defined on Omega with infinite mass boundary conditions. The result is
valid for bounded and unbounded domains and gives estimates on the speed of
convergence. Moreover, the method easily extends when adding external
matrix-valued potentials.
