G.E. Hughes & M.J. Cresswell, A New Introduction to Modal Logic, London and New York: Routledge, 1996, pp. 13-21. (9) Basic modal notions ① In modal logic, there are a number of non-truth-functional concepts. ② Modal Operator L (□) L: necessity operator "necessarily," "must be," "is bound to" Instead of writing "A is bound to be B," we can write "It is bound to be the case that A is B." Lp= Nece..