number for class <code style="white-space: normal">my_class</code> is required,
the value 2 will be returned rather than the default value of 0.
<p>
To diminish typing and enhance readability, a macro is defined
so that instead of the above, we could write:
<pre><code>
BOOST_CLASS_VERSION(my_class, 2)
</code></pre>
which expands to the code above.

<h3><a name="level">Implementation Level</a></h3>
In the same manner as the above, the "level" of implementation of serialization is
specified.  The header file 