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;

theorem
  {x,y} \ X = {x,y} iff not x in X & not y in X by Th48,Th50,XBOOLE_1:83;
