0 for a type, 1 for a name under its type's heading.
OptionalheadingA type split by name: it has no numbers of its own.
Unique within the result, for list keys: the type and, under a
heading, the name, as JSON ('["active"]', '["active","400m"]',
'[null]' for laps with no type).
The laps in it, by id.
OptionalnameThe laps' shared name ('' for none); absent on a heading.
OptionaltypeThe type; undefined for laps with none.
One group, or a heading over a type whose laps carry several names.