Author: Thinker
Revision Date: 2011-01-10 03:24:45 UTC

Fix bug of reference of _doc attribute for PYDocument.

 - PYDocument is a sub-class of PYNode while PYNode ask a PYDocument
   instance, as an argument passed to the constructor, to initialize
   _doc attribute.
 - _doc attribute for a PYDocument instance must be pointed back to
   it-self, but it is still not existed when creating argument list.
 - It is fixed by constructor of PYDocument to point it back to it-self.
   - The caller of constructor just pass a None object, instead of a
     PYDocument instance. PYDocument would fix it later.