  We discuss the derivation of a ``cluster sum rule'' from the
Gellmann-Goldberger-Thirring (GGT) sum rule as an alternative to the
Thomas-Reiche-Kuhn (TRK) sum rule, which was used as the basis up to now. We
compare differences in the assumptions and approximations. Some applications of
the sum rule for halo nuclei, as well as, nuclei with a pronounced cluster
structure are discussed.
