@book{Chellas_1980, address={Cambridge}, title={Modal Logic: An Introduction}, url={http://dx.doi.org/10.1017/CBO9780511621192}, publisher={Cambridge University Press}, author={Chellas, Brian F.}, year={1980} }
@book{G. Winskel_1993, address={Cambridge, Mass}, title={The formal semantics of programming languages}, publisher={MIT Press}, author={G. Winskel}, year={1993} }