code: func (fs *FileSet) PrependLogFile(f *LogFile) *FileSet {
  	return &FileSet{
  		levels: fs.levels,
  		sfile:  fs.sfile,
  		files:  append([]File{f}, fs.files...),
  	}
  }
docs: // PrependLogFile returns a new file set with f added at the beginning.
  // Filters do not need to be rebuilt because log files have no bloom filter.