You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: plain_text_to_markdown.rb
    path of file: ./repos/discourse/lib
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class PlainTextToMarkdown
  SIGNATURE_SEP
    
  def classify_lines
    previous_line = nil

    @lines.each do |line|
      classify_line_as_code!(line, previous_line)

      previous_line = line
    end
  end