Learning COMPUTED BY - what right parenthesis does it need? I am reading this document on Automatic columns.
The first snippet in it creates a COMPUTED BY column:
create table PERSON (
(employee_id integer,
employee_id_disp computed by
SUBSTRING (CAST(employee_id + 100000 as VARCHAR (6)) from 2)
);

When I run this snippet, I get this error:
Error starting at line : 1 in command -
create table PERSON (
(employee_id integer,
employee_id_disp computed by
SUBSTRING (CAST(employee_id + 100000 as VARCHAR (6)) from 2)
)
Error report -
SQL Error: ORA-00904: : invalid identifier
00904. 00000 -  &quot;%s: invalid identifier&quot;
*Cause:    
*Action:

And whey I try to create my own table as follows:
create table A (
  a1 varchar(4000) not null,
  a1Raw computed by UTL_RAW.cast_to_raw(a1)
)

I get a different error:
Error starting at line : 11 in command -
create table A (
  a1 varchar(4000) not null,
  a1Raw computed by UTL_RAW.cast_to_raw(a1)
)
Error report -
SQL Error: ORA-00907: missing right parenthesis
00907. 00000 -  &quot;missing right parenthesis&quot;
*Cause:    
*Action:

I am not overly concerned about the snippet from the Oracle's PDF not working: it sometimes happens that documentation contains bugs. But why is my script not working? I cannot see where a right parenthesis may be possibly missing because all of them match.
Forget COMPUTED BY and just use AS which is the same as GENERATED ALWAYS AS ... VIRTUAL
Syntax summary is in the documentation.
There is no computed by clause and there never has been. OP was looking in the manuals for something called Oracle RDB that runs on OpenVMS and nobody uses.