Kummer structures

Suppose we take an abelian group G and quotient it by the action of negation. What structure does the quotient K inherit from the group structure of G? We describe this structure (which we call the Kummer of G) in terms of a map from the set of unordered pairs of elements of K to itself. We propose some axioms that hold for such structures, and show that every structure satisfying those axioms either is the Kummer of a unique group, or comes from one other construction, the quotient of a 2-torsion group by an involution.