code: def append_allow_trust_op(self,
                                trustor,
                                asset_code,
                                authorize,
                                source=None):
          
          op = operation.AllowTrust(trustor, asset_code, authorize, source)
          return self.append_op(op)
docs: Append an :class:`AllowTrust <stellar_base.operation.AllowTrust>`
          operation to the list of operations.
  
          :param str trustor: The account of the recipient of the trustline.
          :param str asset_code:  The asset of the trustline the source account
              is authorizing. For example, if an anchor wants to allow another
              account to hold its USD credit, the type is USD:anchor.
          :param bool authorize: Flag indicating whether the trustline is
              authorized.
          :param str source: The source address that is establishing the trust in
              the allow trust operation.
          :return: This builder instance.