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Backwards Operator/Retrospective/Hintikka: (Lit: "Rome paper", 1955): "retrospective" operators can make quantified standard modal logic extremely strong.
One can say that the absence of such operators in modal logic is only a coincidence and should be inserted.
Necessity operator/Hintikka: a necessity operator interpreted as standard performs exactly the same as a standard interpreted universal quantifier of a higher level.
Jaakko and Merrill B. Hintikka
The Logic of Epistemology and the Epistemology of Logic Dordrecht 1989
J. Hintikka/M. B. Hintikka
Untersuchungen zu Wittgenstein Frankfurt 1996