let Y be non empty set ; :: thesis: for a being Function of Y,BOOLEAN holds a 'xor' ('not' a) = I_el Y
let a be Function of Y,BOOLEAN; :: thesis: a 'xor' ('not' a) = I_el Y
thus a 'xor' ('not' a) = (('not' a) '&' ('not' a)) 'or' (a '&' ('not' ('not' a))) by BVFUNC_4:9
.= I_el Y by BVFUNC_4:6 ; :: thesis: verum