O Ctx s iff VC. clo]c cs Concrete void push(int v) obj. O Client C int popo t X y: pop( push(x);‖pint; push Abstract pop obi. S… x := 7; push( x ); … … y := pop(); print(y); … Client C Concrete obj. O Abstract obj. S void push(int v) { … } int pop() { … } push pop O ctxt S iff C. C[O]  C[S]