How to fix a field that is set to unused

How to fix a field that is set to unused, the following methods can be recovered (the following steps should be backed up before execution).
1, create the experimental table Ttta
Sql> CREATE TABLE Ttta (A integer,b integer,c VARCHAR2 (A), D INTEGER);
Table has been created.
Sql> INSERT into Ttta VALUES (1, 2, ' 3 ', 4);
1 lines have been created.
Sql> INSERT into Ttta VALUES (2,3, ' 4 ', 5);
1 lines have been created.
Sql> COMMIT;
Submit completed.
ALTER TABLE Ttta SET unused COLUMN C; Set the C field in the Ttta table to be unavailable
2, the following recovery
Logged in as an administrator, where Oracle was the password that was set when oracle10g was installed
Sql> Conn Sys/oracle as Sysdba

Find the number assigned to Ttta in the database
Sql> SELECT obj# from obj$ WHERE name= ' Ttta ';
obj#
----------
32067
SELECT col#,intcol#,name from col$ WHERE obj#=32067;
col# intcol# NAME
---------- ---------- ------------------------------
1 1 A
2 2 B
0 3 sys_c00003_08031720:09:55$ fields to be unused
3 4 D
Sql> SELECT COLS from tab$ WHERE obj#=32067;
COLS
----------
3------The number of fields becomes 3.

sql> UPDATE col$ SET col#=intcol# WHERE obj#=32067;
4 rows have been updated.
sql> UPDATE tab$ SET cols=cols+1 WHERE obj#=32067;
1 rows have been updated.
UPDATE col$ SET name= ' C ' WHERE obj#=32067 and col#=3;
UPDATE col$ SET property=0 WHERE obj#=32067;
Sql> COMMIT; --commits the transaction so that previous actions have been taken into effect
3, restart the database
Sql> SELECT * from SCOTT. Ttta;
A B C D
---------- ---------- ---------- ----------
1 2 3 4
2 3 4 5
Recovery complete