removeBook(bookName) {
        if(!this.books.some(x => x.bookName == bookName)){
            throw new Error(`The book, you're looking for, is not found.`)
        }
        if (this.books.some(x => x.payed == false)) {
            throw new Error(`{bookName} need to be paid before removing from the collection.`)
        }else{
            this.books=this.books.filter(x => x.bookName != bookName);
            return "{bookName} remove from the collection."
        }