  Given a holonomic sequence $F(n)$, we characterize rational functions $r(n)$
so that $r(n)F(n)$ can be summable. We provide upper and lower bounds on the
degree of the numerator of $r(k)$ and show the denominator of $r(n)$ can be
read from annihilators of $F(k)$. This illustration provides the so-called
rational reductions which can be used to generate new multi-sum equalities and
congruences from known ones.
