let Y be non empty set ; :: thesis: for a, b, c being Element of Funcs Y,BOOLEAN holds 'not' ((a '&' b) '&' c) = (('not' a) 'or' ('not' b)) 'or' ('not' c)
let a, b, c be Element of Funcs 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_1:17
.= (('not' a) 'or' ('not' b)) 'or' ('not' c) by BVFUNC_1:17 ;
hence 'not' ((a '&' b) '&' c) = (('not' a) 'or' ('not' b)) 'or' ('not' c) ; :: thesis: verum