_do_comment(self, node) -> None
        Process a comment node. Render a leading or trailing #xA if the
        document order of the comment is greater or lesser (respectively)
        than the document element.

def _do_comment(self, node):
        '''_do_comment(self, node) -> None
        Process a comment node. Render a leading or trailing #xA if the
        document order of the comment is greater or lesser (respectively)
        than the document element.
        '''
        if not _in_subset(self.subset, node): return
        if self.comments:
            W = self.write
            if self.documentOrder == _GreaterElement: W('\n')
            W('<!--')
            W(node.data)
            W('-->')
            if self.documentOrder == _LesserElement: W('\n')