class Array

def product(*arg)
  # Implementation notes: We build a block that will generate all the combinations
  # by building it up successively using "inject" and starting with one
  # responsible to append the values.
  #
  result = []
  arg.map!{|ary| Backports.coerce_to_ary(ary)}
  n = arg.inject(size) { |p, a| p * a.size }
  return [] if n == 0
  raise RangeError, "too big a product" if n > 1<<31
  arg.reverse! # to get the results in the same order as in MRI, vary the last argument first
  arg.push self
  outer_lambda = arg.inject(result.method(:push)) do |proc, values|
    lambda do |partial|
      values.each do |val|
        proc.call(partial.dup << val)
      end
    end
  end
  outer_lambda.call([])
  result
end