I wrote:

>SET TERM ^^ ;
>CREATE TRIGGER TRG_NUMER_NEG FOR TEST_TAB ACTIVE BEFORE INSERT POSITION 0 AS
>begin
> new.NUMER_NEG = new.-NUMER
>end^^
>SET TERM ; ^^

Sorry, I meant

new.NUMER_NEG = -new.NUMER

of course

Set