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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def validate_milestones(self):
        '''
        Reviews the arc element tree to ensure that milestones appear in the right
        order.
        '''
        milestones = self.arc_root_node.get_children().filter(arc_element_type__contains='mile')
        current_cursor = 0
        for mile in milestones:
            seq = mile.milestone_seq
            if seq < current_cursor:
                return mile
            current_cursor = seq
        return None