code: func (trie *Trie) Visit(visitor VisitorFunc) error {
  	return trie.walk(nil, visitor)
  }
docs: // Visit calls visitor on every node containing a non-nil item
  // in alphabetical order.
  //
  // If an error is returned from visitor, the function stops visiting the tree
  // and returns that error, unless it is a special error - SkipSubtree. In that
  // case Visit skips the subtree represented by the current node and continues
  // elsewhere.