Suppose I have a DXL macro with various if/else sections such that a particular string is created only under certain conditions and not other times. How can I test for existence before trying to use said string? I'm looking for the equivalent of 

```
if (exist string foo) {
     do_something
    } else {
     do_some_other_thing
}

```

I could kludge a workaround by initialising `string foo = "notset"` and testing to see if the value was changed, but would prefer something that doesn't require creating the string variable if not needed. 

Note that this is a local string, not something found (or not found) in an attribute.