The foundation is the convention that an empty list can be used to represent the Boolean value <<.value false>> and a list with at one (or more) entries to represent <<.value true>>.
The <<.olink else>> operator can be used to apply a defaults for missing values. In this example, we take advantage of the fact that the <<.olink get>> operator returns an empty list if the field or tiddler does not exist:
The [[:then|Then Filter Run Prefix]] and [[:else|Else Filter Run Prefix]] named filter run prefixes serve a similar purpose as the conditional operators.