// Withdraw from a previous round
  // Cannot withdraw partial funds, all funds are withdrawn
  function withdrawFunds(uint256 roundNum) public {
    require( roundNum > 0 && roundNum < currentRound); // Can only withdraw from previous rounds
    Round storage _round = roundList[roundNum];
    require( _round.roundStatus != RoundStatus.OPEN ); // Round must be closed
    User storage _user = _round.users[msg.sender];
    uint256 balance = _user.callBalanceWei + _user.putBalanceWei;
    require( _user.callBalanceWei + _user.putBalanceWei > 0); // Must have a balance to send out
    _user.callBalanceWei = 0;
    _user.putBalanceWei = 0;
    msg.sender.transfer(balance); // Protected from re-entrancy
  }