Provide a facility to regenerate an Alphabetial index in case of heavily edited
documents.

When large documents are revised, several chapters are added/deleted, and text
is edited heavily. This affects the Index entries entered in the document in the
earlier round. 

This creates a problem for the author: The new/amended text would also have the
same keywords, but the Index cannot show them because the Index entries are not
embedded here.

Writer should have a command to insert the EXISTING Index Entries in amended
parts of the document.

The author could use a concordance file, but then he may not have used a
concordance file the first time; or the file may not be available any longer; or
it may be totally outdated. Therefore, the author should be able to do this
without a concordance file.

Writer should take up all the EXISTING Index Entries and revalidate them.
>> Remove an entry if the amended document does not have the keyword.
>> If the entry is used in new locations, insert it at new locations
>> Ask the user if he wants the entry at each location. Also offer an automatic
mode where user's confirmation is not required.

After this, the user can simply refresh the Index to finish this job.
(For that matter, even this little step can be made part of the Index updation!)