Create a class SongCard that represents a gift card for the purchase of songs online. It should have the following private attributes:

• songs—the number of songs on the card
• activated—true if the card has been activated

and the following methods:

• SongCard(n)—a constructor for a card with n songs.
• activate—activates the gift card. Throws an exception if the card has already been activated.
• buyASong—records the purchase of one song by decreasing the number of songs left for purchase using this card. Throws an exception if the gift card is either completely used or not active.
• songsRemaining—returns the number of songs that can be purchased using the gift card.