reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;

theorem Th4:
  [:X,Y:],[:Y,X:] are_equipotent & card [:X,Y:] = card [:Y,X:] by Lm2;
