let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds a 'nand' (a 'nor' b) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: a 'nand' (a 'nor' b) = I_el Y
thus a 'nand' (a 'nor' b) = (('not' a) 'or' a) 'or' b by Th67
.= (I_el Y) 'or' b by BVFUNC_4:6
.= I_el Y by BVFUNC_1:10 ; :: thesis: verum