let x, y, X be set ; :: thesis: ( {x,y} \ X = {} iff ( x in X & y in X ) )
( {x,y} \ X = {} iff {x,y} c= X ) by XBOOLE_1:37;
hence ( {x,y} \ X = {} iff ( x in X & y in X ) ) by Th38; :: thesis: verum