Murdoch Gabbay Michael Gabbay Substitution for Fraenkel-Mostowski foundations. 65-72 2008 AISB Convention http://www.aisb.org.uk/convention/aisb08/proc/proceedings/12%20Computing%20and%20Philosophy/10.pdf conf/aisb/2008 db/conf/aisb/aisb2008.html#GabbayG08