Luc Steels Remi van Trijp Pieter Wellens Multi-level Selection in the Emergence of Language Systematicity. 425-434 2007 conf/ecal/2007 ECAL http://dx.doi.org/10.1007/978-3-540-74913-4_43 db/conf/ecal/ecal2007.html#SteelsTW07