let Y be non empty set ; :: thesis: for a being Function of Y,BOOLEAN holds (O_el Y) 'nor' a = 'not' a
let a be Function of Y,BOOLEAN; :: thesis: (O_el Y) 'nor' a = 'not' a
thus (O_el Y) 'nor' a = 'not' ((O_el Y) 'or' a) by Th2
.= 'not' a by BVFUNC_1:9 ; :: thesis: verum