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From Wikipedia:

"Modal logic is often referred to as "the logic of necessity and possibility", and such applications continue to play a major role in philosophy of language, epistemology, metaphysics, and formal semantics.[1] However, the mathematical apparatus of modal logic has proved useful in numerous other fields including game theory,[2] moral and legal theory,[2] web design,[2] multiverse-based set theory,[3] and social epistemology.[4] One prominent textbook on the model theory of modal logic suggests that it can be seen more generally as the study of formal systems which take a local perspective on relational structures.[5]"