Each input title from previous runs is passed to this run in turn. The result of each previous call to this run is made available in the next call via the variable named <<.var accumulator>>. The result of the last call to this run is returned as the output. A typical use is to add up the values in a given field of each input title.
Replaces all filter output so far with a single item by repeatedly applying a formula, as described above, to each input title.
<<.tip "Compare named filter run prefix `:reduce` with [[reduce Operator]] which is used to flatten a list of items down to a single item by repeatedly applying a subfilter.">>