TY  - BOOK
AU  - Chellas,Brian F.
TI  - Modal logic: an introduction 
SN  - 0521295157 :
U1  - 160 
PY  - 1980///
CY  - Cambridge
PB  - The University Press
KW  - Modality (Logic)
N1  - Includes bibliographical references and index
ER  -