Hughes, G. E. A New introduction to modal logic / G. E. Hughes, M. J. Cresswell. - London : Routledge, 1996. - x, 421 p. : iill. ; 23 cm. Includes bibliographical references (p. 384-397) and index. ISBN: 0415125995 RM168.96 Subjects--Topical Terms: Modality (Logic).