A BTree is one particular type of index. There are other types, for example
a Hash index. However, all index types have some
basic functionality in common. We've taken this basic index functionality
and created a abstract class called IndexFile. You won't implement
any methods for IndexFile. However, any class derived from an IndexFile
should support IndexFile(), Delete(), and insert(). And you might have to modified the
clause for the Delete() and insert() in IndexFile accordingly
depending on how you should decide to handle the exceptions.
Likewise, an IndexFileScan is a abstract class that contains the basic functionality all index file scans should support.