code: func (l PrimaryLayer) Set(k string, layer SecondaryLayer) {
  	l.lock.Lock()
  	l.store.Put(k, func(oldLayer SecondaryLayer, exists bool) (newLayer SecondaryLayer, write bool) {
  		if exists {
  			oldLayer.Finalize()
  		}
  		return layer, true
  	})
  	l.lock.Unlock()
  }
docs: // Set just adds a secondary layer to the tree, overwriting the previous one.
  // Note that this action would trigger the replaced layer finalizers.