[xml] finding out if XML document had a prolog

[mgrushinskiy comcast net]

Using libxml2 I need to save an XML document with the prolog (<?xml ...>) 
if original document had a prolog and without prolog if original document
didn't have one. It seems that information about xml prolog is not stored
in xmlDoc structure. What is the best way to solve this issue with libxml2?

Thanks,
--MG