export default function(_) {
  return arguments.length
      ? this.cover(+_[0][0], +_[0][1], +_[0][2]).cover(+_[1][0], +_[1][1], +_[1][2])
      : isNaN(this._x0) ? undefined : [[this._x0, this._y0, this._z0], [this._x1, this._y1, this._z1]];
}