function petHouse(){
    class Pet{
        constructor(owner,name) {
            this.owner=owner;
            this.name=name;
            this.comments=[];
        }
        addComment(comment){
            if(this.comments.includes(comment)){
                throw new Error(`This comment is already added!`);
            }
            this, this.comments.push(comment);
            return `Comment is added.`;
        }
        feed(){
            return `${this.name} is fed`
        }

        toString(){
            let result=[];
            result.push(`Here is ${this.owner}'s pet ${this.name}.`)
            if(this.comments.length>0){
               result.push(`Special requirements: ${this.comments.join(", ")}`)
            }
            return result.join("\n");
        }

    }


    class Cat extends Pet{
        constructor(owner, name, insideHabits, scratching) {
            super(owner,name);
            this.insideHabits=insideHabits;
            this.scrathing=scratching;
        }
        feed(){
            return `${super.feed()}, happy and puring.`
        }
        toString(){
            let result=[];
            result.push(`${super.toString()}`);
            result.push(`Main information:`);
            let temp=`${this.name} is a cat with ${this.insideHabits}`;
            if(this.scrathing){
                temp+=`, but beware of scratches.`
            }
            result.push(temp);
            return result.join("\n");
        }
    }

    class Dog extends Pet{
        constructor(owner, name, runningNeeds, trainability) {
            super(owner,name);
            this.runningNeeds=runningNeeds;
            this.trainability=trainability;
        }
        feed(){
            return `${super.feed()}, happy and wagging tail.`;
        }

        toString() {
            let result=[];
            result.push(`${super.toString()}`);
            result.push('Main information:');
            result.push(`${this.name} is a dog with need of ${this.runningNeeds}km running every day and ${this.trainability} trainability.`);
            return result.join('\n');
        }
    }

    return {
        Pet,
        Cat,
        Dog
    }

}