code: func (h *Heap) ListKeys() []string {
  	h.lock.RLock()
  	defer h.lock.RUnlock()
  	list := make([]string, 0, len(h.data.items))
  	for key := range h.data.items {
  		list = append(list, key)
  	}
  	return list
  }
docs: // ListKeys returns a list of all the keys of the objects currently in the Heap.