let x, y, X be set ; :: thesis: ( {x,y} \ X = {x,y} iff ( not x in X & not y in X ) )
( {x,y} \ X = {x,y} iff {x,y} misses X ) by XBOOLE_1:83;
hence ( {x,y} \ X = {x,y} iff ( not x in X & not y in X ) ) by Th55, Th57; :: thesis: verum