@book{28969,
	author = {Hughes, G. E. and Cresswell, M. J. and ebrary, Inc.},
	title = {A new introduction to modal logic},
	publisher = {Routledge,},
	year = {1996.},
	address = {London ;},
	url = {http://site.ebrary.com/lib/rucke/Doc?id=10017164}
}