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


# for example:
class Array
  def molest
    each do |o|
      yield o
    end
  end 
end

[1,2,3,4].molest do |o| 
  puts o + 5 
end