  We reformulate the symmetries of Gurses [Phys. Rev. Lett. 70, 367 (1993)] in
a more abstract, more geometrical manner. The type (b) transformation of
\gurses\ is related to a diffeomorphism of the differentiable manifold onto
itself. The type (c) symmetry is replaced by a more general type (c-bar)
symmetry that has the nice property that the commutator of a type (c-bar)
generator with a type (a) generator is itself of type (c-bar). We identify a
differential constraint that transformations of type (c) and (c-bar) must
satisfy, and which, in our opinion, may severely limit the usefulness of these
transformations.
