def cache_index(index)
      # TODO: atomic rename. Right now we just hope.
      File.open(CACHE_PATH, 'w' ) do |out|
        YAML::dump( index, out )
      end
    end
def cache_index(index)
      # TODO: atomic rename. Right now we just hope.
      File.open(CACHE_PATH, 'w' ) do |out|
        YAML::dump( index, out )
      end
    end
serialize the index to a cache file