  We consider a system of rotators subject to a small quasi-periodic forcing.
We require the forcing to be analytic and satisfy a time-reversibility property
and we assume its frequency vector to be Bryuno. Then we prove that, without
imposing any non-degeneracy condition on the forcing, there exists at least one
quasi-periodic solution with the same frequency vector as the forcing. The
result can be interpreted as a theorem of persistence of lower-dimensional tori
of arbitrary codimension in degenerate cases.
