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
  [:X,{0}:] \/ [:Y,{1}:],[:Y,{0}:] \/ [:X,{1}:] are_equipotent & card([:
  X,{0}:] \/ [:Y,{1}:]) = card([:Y,{0}:] \/ [:X,{1}:]) by Th11;
