[1] G.E. Hughers, M.J. Cress well: An Introduction to Modal Logic. Methuen and Co., 1985.

[2] S.A. Kripke: Semantical considerations on modal logic. Acta Phil. Fenn. XVI, 1963. MR 0170800 | Zbl 0131.00602

[3] C. Lakoff: Hedges: A study in meaning criteria and the logic of fuzzy concepts. J. Philos. Logic 2 1973. MR 0421984 | Zbl 0272.02047

[4] E.J. Lemmon: An Introduction to Modal Logic. American Philosophical Quarterly, Monograph No. 11 ( K. Segerberg, ed.), Oxford 1977. MR 0476365 | Zbl 0388.03006

[5] J.K. Mattila: Proof-theoretical aspects of fuzzy logic. In: Recent Developments in the Theory and Applications of Fuzzy Sets (Bandler and Kandel, eds.), Proceedings of North American Fuzzy Information Processing Society NAFIPS'86 Conference, New Orleans, Lousiana, June 2-4, 1986, pp. 386 - 398.

[6] J.K. Mattila: Calculus of Modifier Operators in Fuzzy Logic. Publications of the Institute for Applied Mathematics No. 14, University of Turku, Turku, Finland 1989. MR 1214143

[7] J. K. Mattila: The construction of a propositional modifier logic. Lappeenranta University of Tech- nology, Department of Information Technology, Research Report No. 21, Lappeenranta, Finland 1990. Zbl 0729.03017