class String
  @@alphabet = ('a'..'z').entries.join('')
  def sum
    letters = split('')
    letters.inject(0) do |sum, letter|
      if (i = @@alphabet.index(letter))
        # puts "#{letter} = #{i}" # Uncommenting this line works
        # sum += i                # ...but uncommenting this line doesn't
      end
    end
  end
end