code: func (c *Corpus) PermanodeModtime(pn blob.Ref) (t time.Time, ok bool) {
  	pm, ok := c.permanodes[pn]
  	if !ok {
  		return
  	}
  
  	// Note: We intentionally don't try to derive any information
  	// (except the owner, elsewhere) from the permanode blob
  	// itself. Even though the permanode blob sometimes has the
  	// GPG signature time, we intentionally ignore it.
  	for _, cl := range pm.Claims {
  		if c.IsDeleted(cl.BlobRef) {
  			continue
  		}
  		if cl.Date.After(t) {
  			t = cl.Date
  		}
  	}
  	return t, !t.IsZero()
  }
docs: // PermanodeModtime returns the latest modification time of the given
  // permanode.
  //
  // The ok value is true only if the permanode is known and has any
  // non-deleted claims. A deleted claim is ignored and neither its
  // claim date nor the date of the delete claim affect the modtime of
  // the permanode.