Overview of RDD System
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     RDD is an abbreviation for Replaceable Database Driver, and it is used
     to describe an interface that controls how your application accesses and
     manipulates database and ancillary files.

     CA-Clipper provides several RDDs to give you access to the database,
     memo, and index file formats of many popular database software products.
     By simply linking the proper RDD with your application, you get
     automatic, easy access to files created by other database engines.

     Moreover, CA-Clipper gives you new and enhanced commands and functions
     designed to make your applications independent of the RDD in use.  Using
     RDDs, you can give end users more flexibility in choosing to migrate to
     your CA-Clipper applications without losing data and to easily move
     their data back and forth between applications if they prefer.