code: func (c *chainView) contains(node *blockNode) bool {
  	return c.nodeByHeight(node.height) == node
  }
docs: // contains returns whether or not the chain view contains the passed block
  // node.  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).