function payFee(bytes32 _serviceName, uint256 _multiplier, address _client) public returns(bool paid) {
        require(isValidService(_serviceName), "_serviceName in invalid");
        require(_multiplier != 0, "_multiplier is zero");
        require(_client != 0, "_client is zero");
        
        require(block.timestamp < nextPaymentTime);

        return true;
    }