function refundEthPrize() external {
        require(waitingEthPrizes[msg.sender] > 0);
        require(address(this).balance >= waitingEthPrizes[msg.sender]);
        uint weiAmountToSend = waitingEthPrizes[msg.sender];
        waitingEthPrizes[msg.sender] = 0;
        msg.sender.transfer(weiAmountToSend);
    }