code: func (node *blockNode) RelativeAncestor(distance int32) *blockNode {
  	return node.Ancestor(node.height - distance)
  }
docs: // RelativeAncestor returns the ancestor block node a relative 'distance' blocks
  // before this node.  This is equivalent to calling Ancestor with the node's
  // height minus provided distance.
  //
  // This function is safe for concurrent access.