code: func (store *store) References(id digest.Digest) []reference.Named {
  	store.mu.RLock()
  	defer store.mu.RUnlock()
  
  	// Convert the internal map to an array for two reasons:
  	// 1) We must not return a mutable
  	// 2) It would be ugly to expose the extraneous map keys to callers.
  
  	var references []reference.Named
  	for _, ref := range store.referencesByIDCache[id] {
  		references = append(references, ref)
  	}
  
  	sort.Sort(lexicalRefs(references))
  
  	return references
  }
docs: // References returns a slice of references to the given ID. The slice
  // will be nil if there are no references to this ID.