public NullChallenge(
     IRankIdentifier rankIdentifier = null, 
     IClashResolver clashResolver = null)
     : base(rankIdentifier, clashResolver)
 {
 }
 protected Challenge(IRankIdentifier rankIdentifier, IClashResolver clashResolver)
 {
     this.clashResolver = clashResolver;
     this.rankIdentifier = rankIdentifier;
     this.nextChallenge = NoChallenge;
 }