title: problem with arrays and operations result beign asaigned to values
body: <p>Here is the python code :</p>
  
  <pre><code>x=[2, 3, 5, 7]
  for i in range(1, 5000):
      if i%2:
          if i%3:
              if i%5:
                  if i%7:
                      x.append(i)
  x.remove(1)
  a = 6
  b = 2
  for i in range(0, 10):
      a = x[a - b]
      b = x[a] - x[b]
  </code></pre>
  
  <p>I get an <code>IndexError: list index out of range</code> for some reason, even though x[] is 1360 and a is just 6 while b is 2 so I dont know why it is happening. Any help would be nice.
  I am using the python shell. Would that be a problem?</p>