let Y be non empty set ; :: thesis: for a, b being Element of Funcs (Y,BOOLEAN) holds a 'or' (a 'nor' b) = a 'or' ('not' b)
let a, b be Element of Funcs (Y,BOOLEAN); :: thesis: a 'or' (a 'nor' b) = a 'or' ('not' b)
thus a 'or' (a 'nor' b) = (a 'or' ('not' a)) '&' (a 'or' ('not' b)) by Th63
.= (I_el Y) '&' (a 'or' ('not' b)) by BVFUNC_4:6
.= a 'or' ('not' b) by BVFUNC_1:9 ; :: thesis: verum