  We provide additional results in connection with Krein's formula, which
describes the resolvent difference of two self-adjoint extensions A_1 and A_2
of a densely defined closed symmetric linear operator A with (possibly
infinite) equal deficiency indices. In particular, we explicitly derive the
linear fractional transformation relating the operator-valued Weyl-Titchmarsh
M-functions M_1(z) and M_2(z) corresponding to A_1 and A_2.
