if (items) {
      if (items instanceof CountingSet) {
        this.#map = new Map(items.#map);
      } else {
        for (const item of items) {
          this.add(item);
        }
      }
    }
A Set that only deletes a given item when the number of delete(item) calls
matches the number of add(item) calls. Iteration and `size` are in terms of
*unique* items.