CREATE EXCEPTION RDB$TEST 'Test';
commit;
set term ^ ;
CREATE OR ALTER TRIGGER RDB$PROCEDURES_BIU FOR RDB$PROCEDURES
ACTIVE AFTER UPDATE OR DELETE POSITION 0
as
begin
 -- if (...) then
    EXCEPTION RDB$TEST;
end^

commit^

create or alter procedure proctest
 returns (result integer)
as
begin
  result = 0;
  suspend;
end
^

set term ; ^

commit;
exit;

FB 1.5:
This operation is not defined for system tables.
unsuccessful metadata update.
MODIFY RDB$PROCEDURES failed.
exception 4.
Test.

FB 2.0: Disconect