Tracking

A per-token update for a single lexer-context field (a "fragment" such as Line or Column).

Tracking.materialize applies one Tracking instance to each case field of the context whose type provides a given, threading the result through a functional copy; fields whose type has no Tracking given are left untouched by the hook (they only change in rule bodies).

This is how tracking composes without inheritance: define a distinct field type and a given Tracking[YourType] in its companion, then use it as a context field.