let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds 'not' (a 'imp' b) = a '&' ('not' b)
let a, b be Function of Y,BOOLEAN; :: thesis: 'not' (a 'imp' b) = a '&' ('not' b)
thus 'not' (a 'imp' b) = 'not' (('not' a) 'or' b) by BVFUNC_4:8
.= ('not' ('not' a)) '&' ('not' b) by BVFUNC_1:13
.= a '&' ('not' b) ; :: thesis: verum