possible to create index of strings and place integer key in it).
Index is implemented using  B+Tree algorithm,  because B+Tree is the most efficient structure
for disk based databases. Methods of <code>Index</code> class allows to add, remove and 
locate objects by key. It is possible to perform search either specifying exact key value either specifying range of key values
(high or low boundary or both of them can be skipped or can be declared as been exclusive or inclusive).
So it is possible to perform t