code: func (c *chainView) next(node *blockNode) *blockNode {
  	if node == nil || !c.contains(node) {
  		return nil
  	}
  
  	return c.nodeByHeight(node.height + 1)
  }
docs: // next returns the successor to the provided node for the chain view.  It will
  // return nil if there is no successor or the provided node is not part of the
  // view.  This only differs from the exported version in that it is up to the
  // caller to ensure the lock is held.
  //
  // See the comment on the exported function for more details.
  //
  // This function MUST be called with the view mutex locked (for reads).