The items, in order. Treat as read-only; mutate through the methods.
Replaces the contents and rebuilds the id index (fixed in v2.4.7, BUGREPORT §2.50).
Whether an item with this id is stored (v2.4.7).
How many items are stored (v2.4.7).
true when an existing item was replaced, false when the id is unknown.
An insertion-ordered array that also keeps an id -> item index.