<a href="../../../boost/serialization/level.hpp" 
target="level_hpp">level.hpp</a> defines the following.
<pre><code>
// names for each level
enum level_type
{
    // Don't serialize this type. An attempt to do so should
    // invoke a compile time assertion.
    not_serializable = 0,
    // write/read this type directly to the archive. In this case
    // serialization code won't be called.  This is the default
    // case for fundamental types.  It presumes a member function or
    // template in the arc