o.mu.RLock()
	defer o.mu.RUnlock()
	return slices.Clone(o.h)
}

// multiIndex stores input and output objects.
// Each input and output can be looked up by its key.
// Additionally, a mapping of input key -> output keys stores the transformation.
type multiIndex[I, O any] struct {
	outputs  map[Key[O]]O
	inputs   map[Key[I]]I
	mappings map[Key[I]]sets.Set[Key[O]]
}