code: function filterSelectors (predicate) {
    if (this._other) {
      const [a, b] = splitFilter(this.selectors, this._other.selectors, predicate);
      this.selectors = a;
      this._other.selectors = b;
    } else {
      this.selectors = this.selectors.filter(predicate);
    }
  }
docs: can be invoked on a style rule to subset its selectors (with reverse mirroring)