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