class Array
  def inject(n)
     each { |value| n = yield(n, value) }
     n
  end
  def sum
    inject(0) { |n, value| n + value }
  end
  def product
    inject(1) { |n, value| n * value }
  end
end
[ 1, 2, 3, 4, 5 ].sum 	» 	15
[ 1, 2, 3, 4, 5 ].product 	» 	120