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: parser.rb
    path of file: ./repos/forem/app/services/html
    
    the code of the file until where you have to start completion: module Html
  class Parser
    # Each of the instance methods should return self to support chaining of
    # methods
    # For example:
    #  Htm
    
    def user_link_if_exists(mention)
      username = mention.delete("@").downcase
      if User.find_by(username: username)
        <<~HTML.chomp
          <a class='mentioned-user' href='#{ApplicationConfig['APP_PROTOCOL']}#{Settings::General.app_domain}/#{username}'>@#{username}</a>
        HTML
      else
        mention
      end