let Y be non empty set ; :: thesis: for a, b being Element of Funcs (Y,BOOLEAN) holds a 'nor' (a 'imp' b) = O_el Y
let a, b be Element of Funcs (Y,BOOLEAN); :: thesis: a 'nor' (a 'imp' b) = O_el Y
thus a 'nor' (a 'imp' b) = (('not' a) '&' a) '&' ('not' b) by Th36
.= (O_el Y) '&' ('not' b) by BVFUNC_4:5
.= O_el Y by BVFUNC_1:5 ; :: thesis: verum