le</span> idx != <span class="py-builtins">len</span>(buf):
        buf[idx] = s2[idx - length]
        idx += 1
    <span class="keyword">return</span> buf
</pre></blockquote>

<p>(You wouldn't really want to do that in Python &mdash; I'm just
explaining what I was thinking.)</p>

<p>Tail-calls are merely "goto with arguments", which means that they
can be implemented very efficiently, and also means that you can
easily use them to create unreadable code.</p>

</li>

<li><b>Native-code compilers get OK per