sentences:  A modal logic with "n" primitive unary modal operators formula_1 is called an "n"-modal logic.Given these operators and negation, one can always add formula_2 modal operators defined as formula_3 if and only if formula_4.Perhaps the first substantive example of a two-modal logic is Arthur Prior's tense logic, with two modalities, F and P, corresponding to "sometime in the future" and "sometime in the past".