All built-in Clojure collections are immutable and heterogeneous, have literal syntax, and support the
conj returns a new collection that is equivalent to an existing collection with an item "added", in either "constant" or logarithmic time. What exactly this means depends on the collection.
count returns the number of items in a collection, in constant time.
nil for an empty collection, or a sequence of items for a non-empty collection, in constant time.