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.

def append_allow_trust_op(self,
                              trustor,
                              asset_code,
                              authorize,
                              source=None):
        """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.

        """
        op = operation.AllowTrust(trustor, asset_code, authorize, source)
        return self.append_op(op)