gistfile1.m

    NSMutableArray * playerIdentifiers = @[].mutableCopy;
    [matches SH_each:^(GKTurnBasedMatch * match) {
      [playerIdentifiers addObjectsFromArray:match.SH_playerIdentifiers];
    }];


    NSArray * playerIdentifiers = [matches SH_reduceValue:@[] withBlock:^id(NSArray * memo, GKTurnBasedMatch * obj) {
      return [memo arrayByAddingObjectsFromArray:obj.SH_playerIdentifiers];
    }];