File: FaultDisputeGame.sol
234:     function step(
235:         uint256 _claimIndex,
236:         bool _isAttack,
237:         bytes calldata _stateData,
238:         bytes calldata _proof
239:     )
240:         public
241:         virtual
242:     {
			 ...
309:         // Set the parent claim as countered. We do not need to append a new claim to the game;
310:         // instead, we can just set the existing parent as countered.
311:         parent.counteredBy = msg.sender;
312:     }
    /// @inheritdoc IFaultDisputeGame
    function step(
        uint256 _claimIndex,
        bool _isAttack,
        bytes calldata _stateData,
        bytes calldata _proof
    )
        public
        virtual
    {
        ...
+       IPreimageOracle oracle = VM.oracle();
+       // Clear the previous preimage proposer if it exists.
+       oracle.clearPreimageProposer();

        bool validStep = VM.step(_stateData, _proof, uuid.raw()) == postState.claim.raw();
        bool parentPostAgree = (parentPos.depth() - postState.position.depth()) % 2 == 0;
        if (parentPostAgree == validStep) revert ValidStep();

        // INVARIANT: A step cannot be made against a claim for a second time.
        if (parent.counteredBy != address(0)) revert DuplicateStep();

        // Set the parent claim as countered. We do not need to append a new claim to the game;
        // instead, we can just set the existing parent as countered.
-       parent.counteredBy = msg.sender;
+       address preimageProposer = oracle.currentPreimageProposer();
+       parent.counteredBy = preimageProposer == address(0) ? msg.sender : preimageProposer;
    }
+   function clearPreimageProposer() external {
+       currentPreimageProposer = address(0);
+   }

-   function readPreimage(bytes32 _key, uint256 _offset) external view returns (bytes32 dat_, uint256 datLen_) {
+   function readPreimage(bytes32 _key, uint256 _offset) external returns (bytes32 dat_, uint256 datLen_) {
        ...
+       currentPreimageProposer = preimageProposer[_key][_offset];
    }
