One that comes immediately to mind is that you might want + to handle vectors as well as scalars. Something like: (define scalar-sum +) (define (+ . a

One that comes immediately to mind is that you might want + to handle vectors as well as scalars. Something like: (define scalar-sum +) (define (+ . arguments) (if (vectors? 'arguments) (vector-sum 'arguments) (apply scalar-sum arguments))) vectors? and vector-sum not shown for clarity. Edit: or maybe you want any sum in excess of (say) $5,000 to alert a manager. (define old-plus +) (define (+ . args) (let ((value (apply old-plus args))) (if (> value 5000) (begin (alert-manager) value) value))) The nice thing here is that this Just Works anywhere + appears in the code. You don't need to put the conditional and the call to alert-manager everywhere you're calculating a sum. Of course, in a real implementation you'd want to pass stuff to alert-manager like what module you're in, the transaction number, etc.