All current serialization wrappers such as <code style="white-space: normal">boost::serialization::nvp</code>,
  <code style="white-space: normal">track_never</code>.
  <li>For all other types, <code style="white-space: normal">track_selectively</code>.
  That is addresses of serialized objects are tracked if and only if
  one or more of the following is true:
  <ul>
  <li>an object of this type is anywhere in the program serialized
  through a pointer.
  <li>the class is explicitly "exported" - see below.
