キャットドア問題が何を問題にしているのかわからない
合力できるようにDoorOpenerCompositionを定義すれば済む話では？

(new Cat(weight: 5)).can_open(new Door(weight: 4, knob: true)) //=> false
(new Human(weight: 15)).can_open(new Door(weight: 14, knob: true) //=>true
(new Cat(weight: 5)).can_open(new Door(weight: 4, knob: false)) //=> true
(new Cat(weight: 5)).can_open(new Door(weight: 10, knob: false)) //=> false
(new Cat(weight: 4) + new Cat(weight: 3) + new Cat(weight: 5)).can_open(new Door(weight: 10, knob: false)) //=> true