code: def is_up_to_date(self):
          
          if self.builder is not MkdirBuilder and not self.exists():
              return 0
          up_to_date = SCons.Node.up_to_date
          for kid in self.children():
              if kid.get_state() > up_to_date:
                  return 0
          return 1
docs: If any child is not up-to-date, then this directory isn't,
          either.