A mutable set of elements of type 'a.

create () returns a new, empty set. It does not allocate any storage until the first element is added: empty sets are cheap.

length t is the number of elements of t. Constant time.

is_empty t is length t = 0.

mem t x is true if x is an element of t.

add t x adds x to t. Does nothing if x is already present.

add_if_new t x adds x to t and returns true if x was already present (in which case t is unchanged). This is the "visited" test of a traversal, in a single lookup.

remove t x removes x from t. Does nothing if x is absent.

clear t removes all elements of t, keeping its storage for reuse.

reset t removes all elements of t and releases its storage: t is then as cheap as a new set.

iter f t applies f to each element of t, in an unspecified order. f can look up t; if it modifies t, it is unspecified whether the added (or removed) elements are visited.

fold f t acc is f xN (... (f x1 acc)), where x1 ... xN are the elements of t, in an unspecified order. Same restrictions as iter.