let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds (('not' a) '&' ('not' b)) 'imp' ('not' (a 'or' b)) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: (('not' a) '&' ('not' b)) 'imp' ('not' (a 'or' b)) = I_el Y
thus (('not' a) '&' ('not' b)) 'imp' ('not' (a 'or' b)) = ('not' (a 'or' b)) 'imp' ('not' (a 'or' b)) by BVFUNC_1:13
.= ('not' (a 'or' b)) 'imp' (('not' a) '&' ('not' b)) by BVFUNC_1:13
.= I_el Y by Th28 ; :: thesis: verum