Enrica Nicolini Christophe Ringeissen Michaël Rusinowitch Combinable Extensions of Abelian Groups. 51-66 2009 CADE http://dx.doi.org/10.1007/978-3-642-02959-2_4 conf/cade/2009 db/conf/cade/cade2009.html#NicoliniRR09