code: func isNegative(n *Node) bool {
  	switch {
  	case n.Flat < 0:
  		return true
  	case n.Flat == 0 && n.Cum < 0:
  		return true
  	default:
  		return false
  	}
  }
docs: // isNegative returns true if the node is considered as "negative" for the
  // purposes of drop_negative.