Stijn Heymans Jos de Bruijn Livia Predoiu Cristina Feier Davy Van Nieuwenborgh Guarded hybrid knowledge bases. 411-429 2008 8 TPLP 3 http://dx.doi.org/10.1017/S1471068407003201 db/journals/tplp/tplp8.html#HeymansBPFN08