sum function returns inconsistent values when used with slicing : <pre><code>lst=[2,5]
sum(lst)
</code></pre>

<p>This returns 7 as expected</p>

<pre><code>lst[0]
</code></pre>

<p>This returns 2 as expected</p>

<pre><code>lst[1]
</code></pre>

<p>This returns 5 as expected</p>

<pre><code>sum(lst[0:-1])
</code></pre>

<p>why does this return 2 instead of 7??</p>
