This a suite of very high-performance immutable-collections

• For lists you should always prefer to use Seq<A> - it is about 10x faster than Lst<A>. The only reason you'd pick Lst<A> is if you needed to do inserts into the middle of the list: Seq<A> doesn't allow this (it only allows prepending or appending), as it would be a performance hit. Seq<A> is backed by an array, and so it has exceptional memory locality, Lst<A> is an AVL tree to allow for efficient
  insertion, but suffers from poorer memory locality.
• For 'dictionaries' or maps as we prefer to call them, then HashMap is the fastest implemention you'll find in .NET-land. It is unsorted. If you need a sorted dictionary, use Map. HashMap uses the CHAMP data-structure, Map uses an AVL tree.
• The same goes for sets, prefer HashSet over Set, unless you need the set to be sorted.