@book{1544,
	author = {Hughes, G. E. and Cresswell, M. J.,},
	title = {An introduction to modal logic},
	publisher = {Methuen},
	year = {[1972, c1968]},
	series = {University paperbacks,},
	address = {London,},
	note = {"Distributed in the U.S.A. by Barnes and Noble, Inc."}
}