title: create an adjustable playlist with ruby
body: <p>I'm not sure why my code will not pass the corresponding test.  Each time I try the code, the following error is reported:  "rb:60:in <code>&lt;main&gt;': undefined local variable or method</code>track' for main:Object (NameError)."  What can I do without editing the tests?  Thanks!  Open to another approach...thanks!
  <strong>strong text</strong></p>
  
  <pre><code>class Song
    attr_reader :song 
    def initialize(song, artist)
      @song = song
      @artist = artist
    end
    def play
     puts "#{@song}by #{@artist}"
    end
  end
  
  class Playlist
    def initialize(player_list)
      @player_list = player_list
    end
  
    def add(add_song)
        add_song.each do |song|
            @player_list &lt;&lt; song
        end
    end
  
    def track_number
        @player_list.length 
    end
  
    def remove(remove_song)
        remove_song.each do |song|
            @player_list.delete(song)
        end
    end
  
    def includes?(from_list)
        i = 0
        from_list.each do |song|
            if @player_list.include?(song) 
                  i+=1
            end
        end
        if i==from_list.length 
          true
        else 
           false
        end
     end
  
    def play_all
        @player_list.each do |song|
              song.play
        end
    end
  
    def display
        @player_list.each do |song|
            puts song.song 
        end
    end
  end
  
  
  
   one_by_one = Song.new("One by One", "Sirenia")
   world_so_cold = Song.new("World So Cold", "Three Days Grace") 
   going_under = Song.new("Going Under", "Evanescence")
  
   my_playlist = Playlist.new(one_by_one, world_so_cold, going_under)
  
   lying_from_you = Song.new("Lying From You", "Linkin Park")
   angels = Song.new("Angels", "Within Temptation")
  
   my_playlist.add(lying_from_you, angels)
   p my_playlist.track_number == 5
   going_under.play
   my_playlist.remove(angels)
   p my_playlist.includes?(lying_from_you) == true
   my_playlist.play_all
   my_playlist.display
  </code></pre>