def fill_prefixes(key, value, prefix, prefixes)
      prefixes = {} if prefixes.nil?
      if prefix.size == 1
        pr = prefix.to_i
        prefixes[pr] ||= {}
        prefixes[pr][key] = value
      else
        pr = prefix[0].to_i
        prefixes[pr] = fill_prefixes(key, value, prefix[1..-1], prefixes[pr])
      end
      prefixes
    end
def fill_prefixes(key, value, prefix, prefixes)
      prefixes = {} if prefixes.nil?
      if prefix.size == 1
        pr = prefix.to_i
        prefixes[pr] ||= {}
        prefixes[pr][key] = value
      else
        pr = prefix[0].to_i
        prefixes[pr] = fill_prefixes(key, value, prefix[1..-1], prefixes[pr])
      end
      prefixes
    end
method updates prefixes hash recursively