code: public static NFSFileVec make(File f, Futures fs) {
      if( !f.exists() ) throw new IllegalArgumentException("File not found: "+f.toString());
      long size = f.length();
      Key k = Vec.newKey(PersistNFS.decodeFile(f));
      // Insert the top-level FileVec key into the store
      NFSFileVec nfs = new NFSFileVec(k,size);
      DKV.put(k,nfs,fs);
      return nfs;
    }
docs: Make a new NFSFileVec key which holds the filename implicitly.  This name
  is used by the Chunks to load data on-demand.
  @return  A NFSFileVec mapped to this file.