token: has $value. reference: has $ref but no $value and no
child members (a JSON-pointer alias, §7.1.2). group: everything else.
Own $type when it is a string (validity is checked separately).
Path segments from the root; the root group has [].
True when this node or an ancestor group declares $extends/$ref, so
properties such as $type may still arrive through group extension.
Nearest ancestor group
$type(string), not including the node itself.