This method is to support tooling that may want to add declarations to
or remove declarations from a Rule. Changes to the list are tracked
so that added declarations are tagged as belonging to this rule, and
the rule for removed declarations is nulled out.
This method is to support tooling that may want to add selectors to
or remove selectors from a Rule. Changes to the list are tracked
so that added selectors are tagged as belonging to this rule, and
the rule for removed selectors is nulled out.