public function revokeTrustline(Asset $asset, $trustorId, $sourceAccountId = null)
    {
        if ($trustorId instanceof Keypair) {
            $trustorId = $trustorId->getPublicKey();
        }

        $op = new AllowTrustOp($asset, new AccountId($trustorId), $sourceAccountId);
        $op->setIsAuthorized(false);

        return $this->addOperation($op);
    }