eference to the Python object somewhere. If
          the Python object is used after the C++ object is destroyed, we'll crash
          Python.
        </p>
<p>
          In keeping with the philosophy that users on the Python side shouldn't
          have to worry about crashing the interpreter, the default behavior is to
          copy the C++ object, and to allow a non-copying behavior only if the user
          writes boost::ref(a1) instead of a1 directly. At least this way, the user
          doesn't 