Each input title from previous runs is passed to this run in turn. The filter run transforms the input titles and the output of this run replaces the input title. For example, the filter run `[get[caption]else{!!title}]` replaces each input title with its caption field, unless the field does not exist in which case the title is preserved.
Note that within the filter run, the <<.var currentTiddler>> variable is set to the title of the tiddler being processed. This permits filter runs like `:map[{!!price}multiply{!!cost}]` to be used for computation.
Filter runs used with the `:map` prefix should return at least the same number of items that they are passed. Input titles for which the filter run returns no output are replaced by an empty string. In particular, when retrieving the value of a field with the [[get Operator]] it is helpful to guard against a missing field value using the [[else Operator]]. For example `[get[myfield]else[default-value]...`.
