# File lib/quiz1/t/solutions/Florian Gross/solitaire.rb, line 178
  def decrypt(raw_text, keystream = nil, pretty = true)
    keystream ||= KeyStream.new
    text = clean(raw_text)

    result = ""
    0.upto(text.size - 1) do |index|
      source_num = letter_to_number(text[index, 1])
      key_num = letter_to_number(keystream.shift)
      result << number_to_letter((source_num - key_num) % 26)
    end

    result = pretty(result) if pretty
    return result
  end