Cf. the Fredkin gate, which has three inputs and three outputs, and in which the number of bits going out equals the number coming in. http://en.wikipedia.org/

Cf. the Fredkin gate, which has three inputs and three outputs, and in which the number of bits going out equals the number coming in. http://en.wikipedia.org/wiki/Fredkin_gate http://en.wikipedia.org/wiki/Fredkin_gate