I would like to know how to create a custom format function for string template. Let say I have the following code:

```
render(attributes) :: <<
<html>
    $atributes: {
        <div> $customformat(atribute.name)$</div>
    }

</html>
>>

customformat(name) ::= <<
    $name; format="upper"$
>>

```

Currently the behaviour of the function customformat is:

Input: "hello world" -> Output: "HELLO WORLD"

And I would like to modify the customformat function so the output is something like the following:

Input: "hello world" -> Output: "HELLO\_WORLD"