  A well-known result by Kemperman describes the structure of those pairs (A,B)
of finite subsets of an abelian group satisfying |A+B|\le|A|+|B|-1. We
establish a description which is, in a sense, dual to Kemperman's, and as an
application sharpen several results due to Deshouillers, Hamidoune, Hennecart,
and Plagne.
