="black">When you pass a list to a function, the function gets a reference
to the list.
If the function modifies a list parameter, the caller sees the change.
For example, <code>delete_head</code> removes the first element from a list:
</font><pre><font size="4" color="blue">
def delete_head(t):
    del t[0]
</font></pre><font color="black">Here's how it is used:
</font><pre><font size="4" color="blue">
&gt;&gt;&gt; letters = ['a', 'b', 'c']
&gt;&gt;&gt; delete_head(letters)
&gt;&gt;&gt; print letters
['b',