function refill(uint256 _value) external {
        require(xrt.transferFrom(msg.sender, this, _value));
        require(_value >= minimalFreeze);

        if (balances[msg.sender] == 0) {
            indexOf[msg.sender] = members.length;
            members.push(msg.sender);
        }
        balances[msg.sender] += _value;
    }