Row grouping can be extracted from the rows themselves by using row grouping on the Zambezi grid directly. But sometimes we might have the groups first, and we would like to gather rows around them, and have them be active even if no rows match.

The gatherRows component is designed for this: you pre-define the groups, and create a predicate for each that will be tested for each row to determine on which group it belongs. Groups are tested in order, and the first one that matches will be used.