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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 add_child(self, valid_policy, qualifier_set, expected_policy_set):
 """ Creates a new PolicyTreeNode as a child of this node :param valid_policy: A unicode string of a policy name or OID :param qualifier_set: An instance of asn1crypto.x509.PolicyQualifierInfos :param expected_policy_set: A set of unicode strings containing policy names or OIDs """


        child = PolicyTreeNode(valid_policy, qualifier_set, expected_policy_set)
        child.parent = self
        self.children.append(child)