    function stop() external auth {
        stopped = true;
    }
    function isClosed() public pure returns (bool closed) {
        return false;
    }
    /// @dev After close_time has been reached, no new offers are allowed.
    modifier can_offer() override {
        require(!isClosed());
        _;
    }

    /// @dev After close, no new buys are allowed.
    modifier can_buy(uint256 id) override {
        require(isActive(id));
        require(!isClosed());
        _;
    }
    // // After close, anyone can cancel an offer
    modifier can_cancel(uint256 id) override {
        require(isActive(id), "Offer was deleted or taken, or never existed.");
        require(
            isClosed() ||
                msg.sender == getOwner(id) ||
                id == dustId ||
                (msg.sender == getRecipient(id) && getOwner(id) == address(0)),
            "Offer can not be cancelled because user is not owner, and market is open, and offer sells required amount of tokens."
        );
        _;
    }
