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 in Z & y in Z implies {x,y} /\ Z = {x,y} by Th31,XBOOLE_1:28;
