# File lib/ruote/util/filter.rb, line 525
  def self.flatten_keys(o, prefix='', accu=[])

    if o.is_a?(Array)

      o.each_with_index do |elt, i|
        pre = "#{prefix}#{i}"
        accu << pre
        flatten_keys(elt, pre + '.', accu)
      end

    elsif o.is_a?(Hash)

      o.keys.sort.each do |key|
        pre = "#{prefix}#{key}"
        accu << pre
        flatten_keys(o[key], pre + '.', accu)
      end
    end

    accu
  end