def DFS_prefix(self, root=None):
if not root:<EOL><INDENT>root = self._root<EOL><DEDENT>return self._DFS_prefix(root)<EOL>
Depth-first search.

.. seealso::
   `Wikipedia DFS descritpion <http://en.wikipedia.org/wiki/Depth-first_search>`_

:param root: first to start the search
:return: list of nodes