How to use spool example

----------main.sql---------------------
spool main.log

prompt --start main--;
prompt ==============;
prompt
prompt ***start sub1.sql***;
@@sub1.sql
spool main.log  append
prompt ***end sub1.sql***;
prompt
prompt ***start sub2.sql***;
@@sub2.sql
spool main.log  append
prompt ***end sub2.sql***;
prompt
prompt ==============;
prompt --end main--;
spool off
------------------------------------
---------sub1.sql-------------------
spool sub1.log
prompt --begin sub1.sql--;
prompt ===================
prompt --do sub1.sql--;
@@sub11.sql
spool sub1.log append
prompt ===================
prompt --end sub1.sql--;
spool off
------------------------------------
----------sub2.sql------------------
spool sub2.log
prompt --do sub2.sql--
spool off
------------------------------------
--------sub11.sql-------------------
spool sub11.log
prompt --do sub11.sql--;
spool off
------------------------------------