SetElementObj: TYPE;
-- Opaque type for a set element object.

SetElement: TYPE = REF SetElementObj;

SetIsDismembered: ERROR;
-- Is raised if any attempt is made to perform an operation on a set that has been effectively
-- destroyed by "DismemberSet," below.

MakeSet: PROC [data: REF ANY] RETURNS [newSetElement: SetElement];
-- Given "data," constructs and returns a new set element representing a singleton set
-- containing the data.

Find: PROC [setElement: SetElement] RETURNS [canonicalElement: SetElement];
-- Given a set element, returns the canonical element of the set containing the given element.
-- Returns NIL if "setElement" is NIL. (! SetIsDismembered)

Union: PROC [setElement1, setElement2: SetElement]
RETURNS [newCanonicalElement: SetElement];
-- Given two set elements, constructs the union set of their respective containing sets and
-- returns the canonical element of the union set. The original sets are effectively destroyed
-- by this process. Note that the given set elements need not be the canonical elements of
-- their containing sets. If both the given elements are contained in the same
-- set or if exactly one of the elements is NIL, the canonical element of the containing set is
-- returned. If both elements are NIL, NIL is returned. (! SetIsDismembered)

GetData: PROC [setElement: SetElement] RETURNS [data: REF ANY];
-- Given a set elements, returns the corresponding data item. Returns NIL if
-- "setElement" is NIL. (! SetIsDismembered)

InSameSet: PROC [setElement1, setElement2: SetElement] RETURNS [equal: BOOLEAN];
-- Given two set elements, returns TRUE if they are in the same set, FALSE otherwise.
-- (! SetIsDismembered)

EnumerateSet: PROC [lastElement: SetElement] RETURNS [nextElement: SetElement];
-- A "stateless enumerator" of set elements. The elements of a set can be thought of as
-- stored in a circular list. Given one set element ("lastElement"), the procedure allows the
-- next one in the same set to be located ("nextElement"). Any set element will do as
-- a starting point, it need not be a canonical element. When the initial element in the
-- enumeration is returned as "nextElement," the enumeration is complete. The order in
-- which elements are enumerated is effectively random. Returns NIL if "lastELement"
-- is NIL. (! SetIsDismembered)

DismemberSet: PROC [setElement: SetElement];
-- Given a set element, effectively destroys its containing set by arranging its data structure
-- so that it may be recovered by the garbage collector. The data structure is marked in
-- such a way that any inadvertent reference to the "dismembered" set will cause the
-- !SetIsDismembered SIGNAL to be raised. (! SetIsDismembered)