code: func (xft *XFastTrie) walkUpNode(root, node, predecessor, successor *node) {
  	n := node.parent
  	for n != nil && n != root {
  		if !isInternal(n.children[1]) && n.children[1] != successor && n.children[1] != node {
  			n.children[1] = successor
  		}
  		if !isInternal(n.children[0]) && n.children[0] != predecessor && n.children[0] != node {
  			n.children[0] = predecessor
  		}
  		n = n.parent
  	}
  }
docs: // walkUpNode will walk up the newly created branch and set predecessor
  // and successor where possible.  If predecessor or successor are nil,
  // this will set nil where possible.