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monadic operation (unary operation) defined on a set S. A function from the domain S into S itself. The identity function is a monadic operation. Other examples are the operations of negation in arithmetic or logic and of taking complements in set theory or in Boolean algebra. Although basically functions, monadic operations are frequently represented using a special notation, e.g. ¬A or A′ or *_A. When the set S is finite, a truth table can be used to define the meaning of the operation.