function releaseOnce() public {
        bytes32 headKey = toKey(msg.sender, 0);
        uint64 head = chains[headKey];
        require(head != 0);
        require(uint64(block.timestamp) > head);
        bytes32 currentKey = toKey(msg.sender, head);

        uint64 next = chains[currentKey];

        uint amount = freezings[currentKey];
        delete freezings[currentKey];

        balances[msg.sender] = balances[msg.sender].add(amount);
        freezingBalance[msg.sender] = freezingBalance[msg.sender].sub(amount);

        if (next == 0) {
            delete chains[headKey];
        } else {
            chains[headKey] = next;
            delete chains[currentKey];
        }
        emit Released(msg.sender, amount);
    }