class Array

def permutation(num = Backports::Undefined)
  return to_enum(:permutation, num) unless block_given?
  num = num.equal?(Backports::Undefined) ?
        size :
        Backports.coerce_to_int(num)
  return self unless (0..size).include? num
  final_lambda = lambda do |partial, remain|
    yield partial
  end
  outer_lambda = (1..num).inject(final_lambda) do |proc, _|
    lambda do |partial, remain|
      remain.each_with_index do |val, i|
        new_remain = remain.dup
        new_remain.delete_at(i)
        proc.call(partial.dup << val, new_remain)
      end
    end
  end
  outer_lambda.call([], dup)
end