theorem
  (line_homography(N)).l1 = (line_homography(N)).l2 implies l1 = l2
  proof
    assume (line_homography(N)).l1 = (line_homography(N)).l2;
    then l1 = (line_homography(N~)).((line_homography(N)).l2) by Th15
           .= l2 by Th15;
    hence thesis;
  end;
