Control Fiels option

In a Table, I have a Boolean Field X.

On X VALIDATE Triger I insert a condition:

IF X = TRUE THEN

A:=0;

Lets supose this:

X default value is FALSE

A= 10;

I Decided change X = TRUE and A becames A=0. Suppose Ichanged my mindand went back tochange theX:=FALSE. How can I say that A:= 10?

Thanks