to determine  when to inject a "&gt;" character at the end of an XML header. XML output archives 
keep their own internal flag indicating that data being written is header data. This
internal flag is reset when an object start tag is written. When 
<code style="white-space: normal">void end_preamble()</code> is invoked and this internal flag is set
a "&gt;" character is appended to the output and the internal flag is reset. The default
implementation for <code style="white-space: normal">void end_preamble()