0 QTRS
app(app(plus, 0), y) → y app(app(plus, app(s, x)), y) → app(s, app(app(plus, x), y)) app(app(sumwith, f), nil) → nil app(app(sumwith, f), app(app(cons, x), xs)) → app(app(plus, app(f, x)), app(app(sumwith, f), xs))