code: func (xft *XFastTrie) walkUpPredecessor(root, node, predecessor *node) {
  	n := predecessor.parent
  	for n != nil && n != root {
  		if !isInternal(n.children[1]) && n.children[1] != predecessor {
  			n.children[1] = node
  		}
  		n = n.parent
  	}
  }
docs: // walkUpPredecessor will walk up the predecessor branch setting
  // the successor where possible.  This breaks when a common
  // ancestor between predecessor and node is found, ie, the root.