let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds a 'nor' (a 'or' b) = 'not' (a 'or' b)
let a, b be Function of Y,BOOLEAN; :: thesis: a 'nor' (a 'or' b) = 'not' (a 'or' b)
thus a 'nor' (a 'or' b) = 'not' ((a 'or' a) 'or' b) by Th34
.= 'not' (a 'or' b) ; :: thesis: verum