Append a :class:`ChangeTrust <stellar_base.operation.ChangeTrust>`
        operation to the list of operations.

        :param str asset_issuer: The issuer address for the asset.
        :param str asset_code: The asset code for the asset.
        :param str limit: The limit of the new trustline.
        :param str source: The source address to add the trustline to.
        :return: This builder instance.

def append_change_trust_op(self, asset_code, asset_issuer, limit=None, source=None):
        """Append a :class:`ChangeTrust <stellar_base.operation.ChangeTrust>`
        operation to the list of operations.

        :param str asset_issuer: The issuer address for the asset.
        :param str asset_code: The asset code for the asset.
        :param str limit: The limit of the new trustline.
        :param str source: The source address to add the trustline to.
        :return: This builder instance.

        """
        asset = Asset(asset_code, asset_issuer)
        op = operation.ChangeTrust(asset, limit, source)
        return self.append_op(op)