  The development of a neutrino moment based radiative-transfer code to
simulate binary neutron-star mergers can easily become an obstacle path because
of the numerous ways in which the solution of the equations may fail. We
describe the implementation of the grey M1 scheme in our fully
general-relativistic magnetohydrodynamics code and detail those choices and
strategies that could lead either to a robust scheme or to a series of
failures. In addition, we present new tests designed to show the consistency
and accuracy of our code in conditions that are similar to realistic merging
conditions and introduce a new, publicly available, benchmark based on the
head-on collision of two neutron stars. This test, which is computationally
less expensive than a complete merging binary but has all the potential
pitfalls of the full scenario, can be used to compare future implementations of
M1 schemes with the one presented here.
