sum

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 68
  def sum(identity = 0)
    if block_given? || !(first.is_a?(Integer) && last.is_a?(Integer))
      super
    else
      actual_last = exclude_end? ? (last - 1) : last
      if actual_last >= first
        (actual_last - first + 1) * (actual_last + first) / 2
      else
        identity
      end
    end
  end