code: func (c *chainView) nodeByHeight(height int32) *blockNode {
  	if height < 0 || height >= int32(len(c.nodes)) {
  		return nil
  	}
  
  	return c.nodes[height]
  }
docs: // nodeByHeight returns the block node at the specified height.  Nil will be
  // returned if the height does not exist.  This only differs from the exported
  // version in that it is up to the caller to ensure the lock is held.
  //
  // This function MUST be called with the view mutex locked (for reads).