Donna is writing a program for her book store that will help her monitor inventory. She has many different kinds of books (fiction, non-fiction, textbooks, cookbooks, etc.) that all require different types of data. She has noticed, however, that several pieces of data are the same no matter which type of book she is selling: title and price. She creates some procedures that report the title and price of a book that she can use with any kind of book she sells. This is an example of what computing principle?