Deque<Element>, a double-ended queue backed by a ring buffer. Deques are range-replaceable, mutable, random-access collections.
OrderedSet<Element>, a variant of the standard Set where the order of items is well-defined and items can be arbitrarily reordered. Uses a ContiguousArray as its backing store, augmented by a separate hash table of bit packed offsets into it.
OrderedDictionary<Key, Value>, an ordered variant of the standard Dictionary, providing similar benefits.