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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 sort_descendants(self, attr="name"):
 """ This function sort the branches of a given tree by considerening node names. After the tree is sorted, nodes are labeled using ascendent numbers. This can be used to ensure that nodes in a tree with the same node names are always labeled in the same way. Note that if duplicated names are present, extra criteria should be added to sort nodes. Unique id is stored as a node._nid attribute """

        node2content = self.get_cached_content(store_attr=attr, container_type=list)
        for n in self.traverse():
            if not n.is_leaf():
                n.children.sort(key=lambda x: str(sorted(node2content[x])))