Recursive hash diff

class Hash
  def diff(hash)
    raise ArgumentError, 'Argument is not a Hash' unless hash.is_a? Hash
    kkeys = []
    self.keys.each do |k|
      diff = self[k].diff(hash[k]) if self[k].is_a? Hash and hash[k].is_a? Hash  
      kkeys << {k => diff} if diff and diff.size > 0
    end
    diff = self.keys - hash.keys 
    kkeys << diff if diff and diff.size > 0
    return kkeys
  end
end