inital commit
gives 21.60 for three different books
books->cart
Extract #discount_for
Fix test names
Pass cart to the constructor
extract base_price
Price booksets
Extract bookset_price
Tests for book sets
Write ALL the codes
Extract VOLUMES (and volumes and volumes...)
Extract #make_booksets
teach array to delete the item we're looking at
 made a bookset class!
 Please enter the commit message for your changes. Lines starting
extract array_extend.rb
Simplify make_one_bookset
Handle 20% discount for 4 books
Make DISCOUNT hash
gives $30 for a full set