code: def iter(self, root=None):
          
          if root is None:
              return iter(self._controls)
          return ((os.path.join(root, outdir), control)
                  for outdir, control in self._controls)
docs: Create an iterator of (directory, control_dict) tuples for all valid
          parameter choices in this :class:`Nest`.
  
          :param root: Root directory
          :rtype: Generator of ``(directory, control_dictionary)`` tuples.