[amibroker] Metastock "PREV" equivalent

["walkegra" <walkegra@xxxxxxxxxxxx>]

Does anyone know how to code the equivalent of Metastock's "PREV" 
function in Amibroker?  

(PREV allows you to create self-referencing formulas. A self 
referencing formula is one that is able to reference the "previous" 
period's value of itself.)