Start with the amount of damage being dealt to the creature and determine what is "lethal." This is usually the creature's toughness minus the amount of damage that it already has marked on it, but ignoring any replacement or prevention effects that will modify this damage
Every time you pay 4 life the next time Library activates it will be giving you more information and better card quality than it otherwise would much like having a shuffle effect
Ok Sabrina's Abra [1st Edition] #94 (Pokemon Gym Challenge) Details All prices are the current market price
They don't generally do anything different to the standard version of the card - a promotional Pikachu and a regular Pikachu have the same effect in battle - but they have a different appearance, artwork, and depending on who you talk to, a certain amount of prestige
The booster display comes with 24 packs, with each booster packs setting you up with 12 cards, including six Common cards, three Uncommon cards, two cards of either Rare, Super Rare, or Legendary rarity, and one random foil card
Oh, and you can force discard against your opponent and they get milled
