  Denote by $\mathcal P(S)$ the semigroup obtained by equipping the family of
all non-empty subsets of a (multiplicatively written) semigroup $S$ with the
operation of setwise multiplication induced by $S$ itself. We call a
subsemigroup $P$ of $\mathcal P(S)$ downward complete if any element of $S$
lies in at least one set $X \in P$ and any non-empty subset of a set in $P$ is
again in $P$.
  We prove, for a commutative semigroup $S$, a characterization of the
cancellative elements of a downward complete subsemigroup of $\mathcal P(S)$ in
terms of the cancellative elements of $S$. Consequently, we show that, if $H$
and $K$ are cancellative semigroups and either of them is commutative, then
every isomorphism from a downward complete subsemigroup of $\mathcal P(H)$ to a
downward complete subsemigroup of $\mathcal P(K)$ restricts to an isomorphism
from $H$ to $K$. This solves an open case of a problem of Tamura and Shafer
from the late 1960s and generalizes a recent result by Bienvenu and
Geroldinger, where it is assumed, among other conditions, that $H$ and $K$ are
numerical monoids.
