Unrecognized Hx.tag directive

Working through the examples, and in the ClickToLoad example, line 30/31, there is the following code:

```
Elem.button [ Hx.get "/"; Hx.tag] [
    Text.raw "Load More Agents..." ] ]
```

Hx.tag doesn't appear to exist, was wondering what the correct Hx directive is, or if the example is unfinished ?