The Adelphi Library needs a program to keep track of all of the items that its customers come to use. Each item is a book, a magazine, or a recording. Each item has a title, a status (either on the shelf, in storage, checked out, or missing) and a "call number" (actually a string of letters, numbers, spaces, and periods). Different kinds of items have different other information: a book has an author and a number of pages; a magazine has a month and year of publication and a number of pages; a recording has a performer and a length in minutes.