model of Idealized Algol raises signi cant problems over and above those associated with the full abstraction problem for PCF [20, 29]. In a previous paper [5, 6], we gave the rst fully abstract model of Idealized Algol with active, i.e. possibly side-e ecting, expressions. It is signi cantly harder to capture the \pure" version of Idealized Algol originally proposed by Reynolds, in which a sha...