code: def add_child(self, valid_policy, qualifier_set, expected_policy_set):
          
  
          child = PolicyTreeNode(valid_policy, qualifier_set, expected_policy_set)
          child.parent = self
          self.children.append(child)
docs: 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