An Example Session

     | ?- db_open('/tmp/db', update, [a(+,-)], '$db_env'(-33470544), DBRef).
     
     DBRef = '$db'(-33470432)
     
     | ?- db_store('$db'(-33470432), a(b,1), _).
     
     | ?- db_store('$db'(-33470432), a(c,2), _).
     
     | ?- db_fetch('$db'(-33470432), a(b,X), _).
     
     X = 1 ? ;
     
     no
     | ?- db_enumerate('$db'(-33470432), X, _).
     
     X = a(b,1) ? ;
     
     X = a(c,2) ? ;
     
     no
     | ?- db_current(DBName, Mode, Spec, EnvRef, DBRef).
     
     Mode = update,
     Spec = [a(+,-)],
     DBRef = '$db'(-33470432),
     DBName = '/tmp/db',
     EnvRef = '$db_env'(-33470544) ? ;
     
     no
     | ?- db_close_env('$db_env'(-33470544)).