    function merge(uint256 _from, uint256 _to) external {
        require(attachments[_from] == 0 && !voted[_from], 'attached');
        require(_from != _to);
        require(_isApprovedOrOwner(msg.sender, _from));
        require(_isApprovedOrOwner(msg.sender, _to));

        LockedBalance memory _locked0 = locked[_from];
        LockedBalance memory _locked1 = locked[_to];
        uint256 value0 = uint256(int256(_locked0.amount));
        uint256 end = _locked0.end >= _locked1.end ? _locked0.end : _locked1.end;

        locked[_from] = LockedBalance(0, 0);
        _checkpoint(_from, _locked0, LockedBalance(0, 0));
        _burn(_from);
        _deposit_for(_to, value0, end, _locked1, DepositType.MERGE_TYPE);
    }
    /// @notice Withdraw all tokens for `_tokenId`
    /// @dev Only possible if the lock has expired
    function withdraw(uint256 _tokenId) external nonreentrant {
        assert(_isApprovedOrOwner(msg.sender, _tokenId));
        require(attachments[_tokenId] == 0 && !voted[_tokenId], 'attached');

        LockedBalance memory _locked = locked[_tokenId];
        require(block.timestamp >= _locked.end, "The lock didn't expire");
        uint256 value = uint256(int256(_locked.amount));

        locked[_tokenId] = LockedBalance(0, 0);
        uint256 supply_before = supply;
        supply = supply_before - value;

        // old_locked can have either expired <= timestamp or zero end
        // _locked has only 0 end
        // Both can have >= 0 amount
        _checkpoint(_tokenId, _locked, LockedBalance(0, 0));

        assert(IERC20(token).transfer(msg.sender, value));

        // Burn the NFT
        _burn(_tokenId);

        emit Withdraw(msg.sender, _tokenId, value, block.timestamp);
        emit Supply(supply_before, supply_before - value);
    }
    function _burn(uint256 _tokenId) internal {
        require(_isApprovedOrOwner(msg.sender, _tokenId), 'caller is not owner nor approved');

        address owner = ownerOf(_tokenId);

        // Clear approval
        approve(address(0), _tokenId);
        // Remove token
        _removeTokenFrom(msg.sender, _tokenId);
        emit Transfer(owner, address(0), _tokenId);
    }
    function _burn(uint256 _tokenId) internal {
        require(_isApprovedOrOwner(msg.sender, _tokenId), 'caller is not owner nor approved');

        address owner = ownerOf(_tokenId);

        // Clear approval
        approve(address(0), _tokenId);
        // Remove token
        _removeTokenFrom(owner, _tokenId);
        emit Transfer(owner, address(0), _tokenId);
    }
