Navigation Buttons

 
I have these buttons on a form.  The arrows are created using the Windows+; option ''

Under the two right buttons in 'Display if' I have:

let lastId := last((select INVENTORY).ID);
if this.ID = lastId then false else true end

and

in the two left buttons I have:

let firstId := first((select INVENTORY).ID);
if this.ID = firstId then false else true end

Result being that if at the last the two right vanish and so on.