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 Th87:
  [x,y] in [:X,Y:] implies [y,x] in [:Y,X:]
proof
  [x,y] in [:X,Y:] implies x in X & y in Y by Lm16;
  hence thesis by Lm16;
end;
