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ALGO :
this script use an $op to know what to do.
if $op is empty or none of the values listed below,
- the default screen is built (with all or filtered (if search string is set) records).
- the user can click on add, modify or delete record.
- filtering is done on the code field
if $op=add_form
- if primary key (module + code) exists, this is a modification,so we read the required record
- builds the add/modify form
if $op=add_validate
- the user has just send data, so we create/modify the record
if $op=delete_form
- we show the record selected and ask for confirmation
if $op=delete_confirm
- we delete the designated record
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