code: func (l PermalinkExpander) Expand(key string, p Page) (string, error) {
  	expand, found := l.expanders[key]
  
  	if !found {
  		return "", nil
  	}
  
  	return expand(p)
  
  }
docs: // Expand expands the path in p according to the rules defined for the given key.
  // If no rules are found for the given key, an empty string is returned.