  It is shown that the Ginsparg-Wilson relation implies an exact symmetry of
the fermion action, which may be regarded as a lattice form of an infinitesimal
chiral rotation. Using this result it is straightforward to construct lattice
Yukawa models with unbroken flavour and chiral symmetries and no doubling of
the fermion spectrum. A contradiction with the Nielsen-Ninomiya theorem is
avoided, because the chiral symmetry is realized in a different way than has
been assumed when proving the theorem.
