let x, y, X, Y be set ; :: thesis: ( [x,y] in [:X,Y:] implies [y,x] in [:Y,X:] )
( [x,y] in [:X,Y:] implies ( x in X & y in Y ) ) by Lm17;
hence ( [x,y] in [:X,Y:] implies [y,x] in [:Y,X:] ) by Lm17; :: thesis: verum