List of IPython Notebooks by Peter Norvig
In the linked notebook <a href="http://nbviewer.jupyter.org/url/norvig.com/ipython/Countdown.ipynb" rel="nofollow">http://nbviewer.jupyter.org/url/norvig.com/ipython/Countdown...</a> when he says<p>>because there are 2 ways to make 27 with the numbers (10, 9, 8), namely, ((10+9)+8) and (10+(9+8))<p>why not also (10+9+8)?