  Let A be a finite nonempty subset of an additive abelian group G, and let
\Sigma(A) denote the set of all group elements representable as a sum of some
subset of A. We prove that |\Sigma(A)| >= |H| + 1/64 |A H|^2 where H is the
stabilizer of \Sigma(A). Our result implies that \Sigma(A) = Z/nZ for every set
A of units of Z/nZ with |A| >= 8 \sqrt{n}. This consequence was first proved by
Erd\H{o}s and Heilbronn for n prime, and by Vu (with a weaker constant) for
general n.
