The groups one level down, in the order their first records appear.
The row of the group's first record within the keys.
The keys, one per level down to this group's own. Each level's key is spelled by the first
record of the group at that level, so a group's keys begin with its parent's. A blank key is
null.
The number of key columns the group is keyed by: 0 for the root, 1 for the groups of the
first key column, and so on.
The group one level up, or null for the root.
The number of kept records in the group.
One group of a PivotAxis. The axis answers the same object for a group every time, so a group can key a map.