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