sum

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 126
  def sum(identity = nil)
    if block_given? || !(first.is_a?(Integer) && last.is_a?(Integer))
      super
    else
      actual_last = exclude_end? ? (last - 1) : last
      if actual_last >= first
        sum = identity || 0
        sum + (actual_last - first + 1) * (actual_last + first) / 2
      else
        identity || 0
      end
    end
  end