code: public FileMetaData input(int which, int i)
      {
          checkArgument(which == 0 || which == 1, "which must be either 0 or 1");
          if (which == 0) {
              return levelInputs.get(i);
          }
          else {
              return levelUpInputs.get(i);
          }
      }
docs: Return the ith input file at "level()+which" ("which" must be 0 or 1).