Adds the given item with the given priority, if this container is still in the accumulation phase, and returns a handle to remove it. If the container is already in the draining phase, the item is not added and instead the draining phase label is returned.

The same item can be added multiple times, with the same or different priorities, and the container then holds it multiple times. Accordingly, the item will appear as many times during draining as it was added and not removed, unless it has become obsolete.

Hook for testing interesting interleavings

Returns an iterator over the items to be drained in priority order. Lower priority values are drained first.

May only be called during the draining phase.

An item is to be drained if all of the following conditions apply:

• It was added during the accumulation phase.
• It has not yet been successfully removed via the handle.
• It has not yet been garbage-collected because it was obsolete at some point since it was added.

If called multiple times during the draining phase, each returned iterator yields only a subset of the items to be drained, respecting the priority order within the subset. All these iterators jointly return all items to be drained with their correct multiplicity.

The returned iterator is not thread-safe: it must not be used from multiple threads without explicit synchronization.

Switches from the accumulation phase to the draining phase. Only the first call to this method is effective; all subsequent calls are ignored.