public IEnumerable <PlayerNumber> NextMatchCandidates()
{
var lastDefeatedRound = Rounds
.OrderByDescending(it => it.Number)
.Select(it => (Round?)it)
.FirstOrDefault(it => it.Value.HasDefeateds);
var targetRounds = Rounds.SkipWhile(it => it.Number <= (lastDefeatedRound?.Number ?? 0));
var alivedOthers = AlivedOthers();
var recentRoundCount = 4 < alivedOthers.Count ? 4 : alivedOthers.Count - 1;
var recentRounds = targetRounds.OrderByDescending(it => it.Number).Take(recentRoundCount);
return(alivedOthers.Except(recentRounds.Select(it => it.MatchedOther)));
}