create table T (
    ID    integer not null,
    VAL   integer not null,
    CALC  computed by ( VAL + 1 )
);

alter table T
  add constraint PK_T
      primary key ( ID );

insert into T ( ID, VAL ) values ( 1, 2 );
insert into T ( ID, VAL ) values ( 2, 3 );

commit;

create view V (
    ID,
    VAL,
    CALC )
as
select ID,
       VAL,
       CALC
  from T
;

set term ^ ;

create trigger V_BU for V
as
begin
  if ( new.CALC is distinct from old.CALC ) then
    new.VAL = new.CALC - 1;

  update T
     set VAL = new.VAL
   where ID = old.ID;
end ^

set term ; ^

update V
   set CALC = 2
 where ID = 2;

commit;

create trigger V_BU for V
active before update position 0
as
begin
  if ( new.CALC is distinct from old.CALC ) then
    new.VAL = new.CALC - 1;

  update T
     set VAL = new.VAL
   where ID = old.ID;
end