Murdoch James Gabbay A study of substitution, using nominal techniques and Fraenkel-Mostowksi sets. 1159-1189 2009 410 Theor. Comput. Sci. 12-13 http://dx.doi.org/10.1016/j.tcs.2008.11.013 db/journals/tcs/tcs410.html#Gabbay09