let Y be non empty set ; :: thesis: for a, b, c being Function of Y,BOOLEAN holds a 'imp' (b 'nor' c) = 'not' (a '&' (b 'or' c))
let a, b, c be Function of Y,BOOLEAN; :: thesis: a 'imp' (b 'nor' c) = 'not' (a '&' (b 'or' c))
thus a 'imp' (b 'nor' c) = a 'imp' ('not' (b 'or' c)) by Th2
.= ('not' a) 'or' ('not' (b 'or' c)) by BVFUNC_4:8
.= 'not' (a '&' (b 'or' c)) by BVFUNC_1:14 ; :: thesis: verum