let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds 'not' (a 'nor' b) = a 'or' b
let a, b be Function of Y,BOOLEAN; :: thesis: 'not' (a 'nor' b) = a 'or' b
'not' (a 'nor' b) = 'not' ('not' (a 'or' b)) by Th2;
hence 'not' (a 'nor' b) = a 'or' b ; :: thesis: verum