code: def getDefaultSource(self):
          
          xrefs = {}
          for xref in self.grammar.getElementsByTagName("xref"):
              xrefs[xref.attributes["id"].value] = 1
          xrefs = xrefs.keys()
          standaloneXrefs = [e for e in self.refs.keys() if e not in xrefs]
          if not standaloneXrefs:
              raise NoSourceError, "can't guess source, and no source specified"
          return '<xref id="%s"/>' % random.choice(standaloneXrefs)
docs: guess default source of the current grammar
          
          The default source will be one of the <ref>s that is not
          cross-referenced.  This sounds complicated but it's not.
          Example: The default source for kant.xml is
          "<xref id='section'/>", because 'section' is the one <ref>
          that is not <xref>'d anywhere in the grammar.
          In most grammars, the default source will produce the
          longest (and most interesting) output.