title: Why does Python start at index -1 (as opposed to 0) when indexing a list from the end?
body: <pre class="lang-py prettyprint-override"><code>list = ["a", "b", "c", "d"]
  print(list[3]) # Number 3 is "d"
  
  print(list[-4]) # Number -4 is "a"
  </code></pre>