code: func (c *Corpus) mergeMediaTag(k, v []byte) error {
  	f := strings.Split(string(k), "|")
  	if len(f) != 3 {
  		return fmt.Errorf("unexpected key %q", k)
  	}
  	wholeRef, ok := blob.Parse(f[1])
  	if !ok {
  		return fmt.Errorf("failed to parse wholeref from key %q", k)
  	}
  	tm, ok := c.mediaTags[wholeRef]
  	if !ok {
  		tm = make(map[string]string)
  		c.mediaTags[wholeRef] = tm
  	}
  	tm[c.str(f[2])] = c.str(urld(string(v)))
  	return nil
  }
docs: // "mediatag|sha1-2b219be9d9691b4f8090e7ee2690098097f59566|album" = "Some+Album+Name"