code: function (el) {
      var index = this.usedEls.indexOf(el);
      if (index === -1) {
        warn('The returned entity was not previously pooled from ' + this.attrName);
        return;
      }
      this.usedEls.splice(index, 1);
      this.availableEls.push(el);
      el.object3D.visible = false;
      el.pause();
      return el;
    }
docs: Used to return a used entity to the pool.