corner cases for next and previous

hws/spotify_playlists/README.md

@@ -105,8 +105,8 @@ The **actions file** may include 5 different types of actions:
 - add, which adds a song to the end of the playlist.
 - remove, which removes a song from the playlist.
 - move, which moves a song to a new position - the new position is always included at the end of the line. The line *move "I Will Never Love Again - Film Version" Lady Gaga, Bradley Cooper 1*, moves the song "I Will Never Love Again - Film Version" to position 1, and the line *move "You Are Still the One" Shania Twain 4*, moves the song "You Are Still the One" to position 4. Note that, unliked array indexing in C/C++, positioning in Spotify starts at 1, as opposed to 0. This can be seen in the above Spotify screenshot: the first position is position 1.
-- next, which skips the currently playing song and starts playing the song that is listed directly after it.
-- previous, which skips the currently playing song and goes to the song listed directly before the currently playing song.
+- next, which skips the currently playing song and starts playing the song that is listed directly after it. Note that if the currently playing song is already at the bottom of the playlist, the action *next* will make the first song (i.e., the song at the very top of the playlist) as the currently playing song.
+- previous, which skips the currently playing song and goes to the song listed directly before the currently playing song. Note that if the currently playing song is already at the top of the playlist, the action *previous* will make the last song (i.e., the song at the bottom of the playlist) as the currently playing song.
 
 According to this sample **actions file**, 4 songs will be added to the playlist, 1 song will be removed, 2 songs will be moved. And the currently playing song will be a different song, instead of the song "Always Remember Us This Way".