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 Th13:
  card(A*^B) = card A *` card B
proof
  thus card (A*^B) = card [:A,B:] by Th10
    .= card A *` card B by Th6;
end;
