code: func (n *node) prevSibling() *node {
  	if n.parent == nil {
  		return nil
  	}
  	index := n.parent.childIndex(n)
  	if index == 0 {
  		return nil
  	}
  	return n.parent.childAt(index - 1)
  }
docs: // prevSibling returns the previous node with the same parent.