At present, it doesn't seem to be possible to format fields containing vars according to whether a var is set or not. For examples, I would like to include some of the location / event-related vars to generate a default description but not include the following comma if the var is undefined.
One way of achieving this would be to make it possible to say "{event, }", for example, and if the var {event} is unset, then the ", " would not be macro expanded either.
There are some design questions, such as "how do you recognise var names within {}?" (Eg, the var name is the first contiguous sequence of lower-case alpha chars within the {}" but you could be more restrictive.
