sqlplus: create database

hello,

sorry for my english. I try to create manually a database in Oracle 9i. I don't understand this step:

CREATE TABLESPACE rollback DATAFILE '....' ....

/***************/

CREATE ROLLBACK SEGMENT t0;
ALTER ROLLBACK SEGMENT t0 ONLINE;
SET TRANSACTION USE ROLLBACK SEGMENT t0; CREATE ROLLBACK SEGMENT r0 TABLESPACE rollback; CREATE ROLLBACK SEGMENT r1 TABLESPACE rollback; ALTER ROLLBACK SEGMENT t0 OFFLINE;
DROP ROLLBACK SEGMENT t0;
/***************/

if I try to create directly the rollback segments r0 - r1 I have an error !! why ??

tanks in advance ...

-- 
#include <stdio.h>
int main(void){char c[]={10,65,110,116,111,110,105,111,32,98,97,114,98,111,110,
101,32,60,104,105,110,100,101,109,105,116,64,116,105,115,99,97,108,105,110,101,
116,46,105,116,62,10,10,0};printf("%s",c);return 0;}