an input-consuming logic gate "l" is reversible if, for any output "y", there is a unique input "x" such that applying "l"("x") = "y". if a gate "l" is reversible, there is an inverse gate "l"′, which maps "y" to "x" for which "l"′("y") = "x". from common logic gates, not is reversible, as can be seen from its truth table below.