code: func (c *PageCollections) getFromCache(ref string) (page.Page, error) {
  	v, found, err := c.pageIndex.Get(ref)
  	if err != nil {
  		return nil, err
  	}
  	if !found {
  		return nil, nil
  	}
  
  	p := v.(page.Page)
  
  	if p != ambiguityFlag {
  		return p, nil
  	}
  	return nil, fmt.Errorf("page reference %q is ambiguous", ref)
  }
docs: // Get initializes the index if not already done so, then
  // looks up the given page ref, returns nil if no value found.