Shelf implementation using the "BSD" db interface.
This adds methods first(), next(), previous(), last() and
set_location() that have no counterpart in [g]dbm databases.
The actual database must be opened using one of the "bsddb"
modules "open" routines (i.e. bsddb.hashopen, bsddb.btopen or
bsddb.rnopen) and passed to the constructor.
See the module's __doc__ string for an overview of the interface.