reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;
reserve e for object, X,X1,X2,Y1,Y2 for set;

theorem
  x <> y implies A \/ {x} \ {y} = A \ {y} \/ {x} by Th11,XBOOLE_1:87;
