Efficient way to create/copy immutable struct instance with modified data

[Merlin Diavova]

Hi all,

I'm trying to figure out the most efficient way to create 
modified instances of immutable structs.

Currently, I'm doing the following:

```d
immutable struct Node {
	string label;
	Node parentNode;
	NetworkPort port;

	auto withLabel(string newLabel)
	{
		return Node(newLabel, this.parentNode, this.port);
	}

	auto withParentNode(Node newParentNode)
	{
		return Node(this.label, newParentNode, this.port);
	}

	auto withNetworkPort(NetworkPort newPort)
	{
		return Node(this.label, this.parentNode, newPort);
	}
}
```
Coming from a scripting language the above makes the most sense.
In D, this an efficient way to do it? Are there any best 
practices around this?

Thanks in advance

Merlin