let Y be non empty set ; :: thesis: for a, b, c being Function of Y,BOOLEAN holds a 'or' (b 'nor' c) = (a 'or' ('not' b)) '&' (a 'or' ('not' c))
let a, b, c be Function of Y,BOOLEAN; :: thesis: a 'or' (b 'nor' c) = (a 'or' ('not' b)) '&' (a 'or' ('not' c))
thus a 'or' (b 'nor' c) = a 'or' ('not' (b 'or' c)) by Th2
.= a 'or' (('not' b) '&' ('not' c)) by BVFUNC_1:13
.= (a 'or' ('not' b)) '&' (a 'or' ('not' c)) by BVFUNC_1:11 ; :: thesis: verum