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 Th21:
  (K*`M)*`N = K*`(M*`N)
proof
  thus (K*`M)*`N = card [:[:K,M:],N:] by Th6
    .= card [:K,[:M,N:]:] by Th5
    .= K*`(M*`N) by Th6;
end;
