An Anchor tracks a groups as its index changes due to other groups being inserted and
removed before it. If the group the Anchor is tracking is removed, directly or indirectly,
valid will return false. The current index of the group can be determined by passing either
the SlotTable or SlotWriter to toIndexFor. If a SlotWriter is active, it must be used
instead of the SlotTable as the anchor index could have shifted due to operations performed
on the writer.