let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds (('not' a) 'or' ('not' b)) 'imp' ('not' (a '&' b)) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: (('not' a) 'or' ('not' b)) 'imp' ('not' (a '&' b)) = I_el Y
thus (('not' a) 'or' ('not' b)) 'imp' ('not' (a '&' b)) = ('not' (a '&' b)) 'imp' ('not' (a '&' b)) by BVFUNC_1:14
.= ('not' (a '&' b)) 'imp' (('not' a) 'or' ('not' b)) by BVFUNC_1:14
.= I_el Y by Th36 ; :: thesis: verum