Method add

src/utils/SimpleSet.js:11–14

(k)

    return this.v.indexOf(k) !== -1;
  }
  add(k) {
    if (this.has(k)) return;
    this.v.push(k);
  }
  delete(k) {
    const idx = this.v.indexOf(k);
    if (idx === -1) return false;