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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 _clean(self):
 """Recursively check that all nodes are reachable."""

        found_ids = {}
        nodes = [self._nodes[_node.Root.ID]]
        while nodes:
            node = nodes.pop()
            found_ids[node.id] = None
            nodes = nodes + node.children

        for node_id in self._nodes:
            if node_id in found_ids:
                continue
            logger.error('Dangling node: %s', node_id)

        for node_id in found_ids:
            if node_id in self._nodes:
                continue
            logger.error('Unregistered node: %s', node_id)