This is the Smart Contract used in the paper "Regulating Blockchain Smart Contracts WithAgent-Based Markets"

contract.sol

pragma solidity >=0.4.4 <0.6.0;

contract BuyItemContract {
    string item = "myItemKey";
    uint price;
    address payable seller;

    event Sell(address seller, string item);
    event Sold(address buyer, string item, uint value);
    event Received(address seller, uint price);

    constructor() public payable {
    }

    function() external payable {}

    function setSeller() public {
        seller = msg.sender;
        emit Sell(seller, item);
    }

    function payHalf(uint value) public payable returns (bool) {
        if (msg.value != 0) {
            value = msg.value;
        }
        if (value < 100) {
            revert();
        } else {
            price += value;
            emit Sold(msg.sender, item, price);
        }
        return true;
    }

    function payOtherHalf(uint value) public payable {
        if (msg.value != 0) {
            value = msg.value;
        }
        if (value != 2 * price) {
            revert();
        } else {
            seller.transfer(2 * price);
            emit Received(seller, 2 * price);
        }
    }

    function expectedOtherHalf() public view returns (uint) {
        return (2 * price);
    }
}