  Given a finite subset A of an abelian group G, we study the set k \wedge A of
all sums of k distinct elements of A. In this paper, we prove that |k \wedge A|
>= |A| for all k in {2,...,|A|-2}, unless k is in {2,|A|-2} and A is a coset of
an elementary 2-subgroup of G. Furthermore, we characterize those finite
subsets A of G for which |k \wedge A| = |A| for some k in {2,...,|A|-2}. This
result answers a question of Diderrich. Our proof relies on an elementary
property of proper edge-colourings of the complete graph.
