sum

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 122
  def sum(identity = 0)
    return super if block_given? || !(first.instance_of?(Integer) && last.instance_of?(Integer))
    actual_last = exclude_end? ? (last - 1) : last
    (actual_last - first + 1) * (actual_last + first) / 2
  end