That should be OK, it's an immutable one.

![image](https://f.cloud.github.com/assets/287532/1104909/7ae04772-1910-11e3-8f1e-46c5dd12daba.png)

If we consider using F# idiomatically, this looks like the way to do this particular task (that is, an explicit constructor with a Guard Clause and an assignment to a read-only member).

-----

Please note that I tried renaming the field to `_relay` and create a C#-like read-only property:
```f#
member this.Relay 
    with get() = _relay
```
But then, the F# compiler didn't let me go through:

![image](https://f.cloud.github.com/assets/287532/1104937/77daf44e-1912-11e3-94d8-5b04f346a005.png)