(self)
Return a pre-order iterator for the tree.

        This must be implemented by the concrete subclass.
Return a pre-order iterator for the tree.
Return a pre - order iterator for the tree .
def pre_order(self):
        """
        Return a pre-order iterator for the tree.

        This must be implemented by the concrete subclass.
        """
        raise NotImplementedError
def pre_order ( self ) : raise NotImplementedError