let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds a 'nand' (a '&' b) = a 'nand' b
let a, b be Function of Y,BOOLEAN; :: thesis: a 'nand' (a '&' b) = a 'nand' b
a 'nand' (a '&' b) = (a '&' a) 'nand' b by Th10;
hence a 'nand' (a '&' b) = a 'nand' b ; :: thesis: verum