gistfile1.m

  NSMutableArray * setOfPlayerIds = @[].mutableCopy;
   for(GKTurnBasedMatch * match in theMatches) {
    [setOfPlayerIds addObjectsFromArray:match.SH_playerIdentifiers];
  }
  [setOfPlayerIds addObjectsFromArray:theFriends];


//Immutable
setOfPlayerIds = [theMatches SH_reduceValue:theFriends withBlock:^id(NSArray * memo, GKTurnBasedMatch * obj) {
    return [memo arrayByAddingObjectsFromArray:obj.SH_playerIdentifiers];
  }];