THIS IS NOT A PROPOSAL. It's just a dumping ground for some stuff I don't want to lose track of. There will be a proper proposal later.
Based on unique objects, not symbols.
Each enum object has four properties: enum type, name (a symbol), ordinal (a non-negative exact integer), value (anything). Enum objects are unique across enum types. Names and ordinals are unique within an enum type. Accessors are enum-type, enum-name, enum-ordinal, enum-value.
An enum type contains a map from names to enum objects and a vector indexed by the ordinal. We need enum-ordinal->enum (uses the vector) and enum-name->enum (uses the map) as fundamental procedures on enum types. Simple composition gives us enum-name->ordinal, enum-ordinal->name, enum-ordinal->value, and enum->name->value. There is no way to work backward from an enum value because it need not be unique. To retrieve all enums, we have enum-type-enums and enum-type-names.
Having a single value is very handy for C definitions, etc.
Make-enumeration-type accepts either names or (name value) lists. Ordinals are assigned in order. Define-enumeration-type binds an identifier to the enumeration type and an identifier per enum (same as the name) to each enum.
Except as noted below, the procedures for creating and manipulating enumeration sets are the same as those for sets, except that set is replaced by enum-set in their names. Wherever a newly allocated enumeration set is returned, it has the same enumeration type as the source sets. It is an error to operate on enumeration sets of different types in a single procedure.
(make-universal-enum-set enum-type)
Returns a newly allocated enumeration set. The possible elements of the set are the enum objects in enum-type. The set contains all possible elements. The approximate R6RS equivalent is enum-set-universe.
(enum-set enum-type element ...)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set is initialized to contain the elements. There is no R6RS equivalent.
(list->enum-set enum-type list)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set is initialized to contain the elements of list. There is no R6RS equivalent.
(enum-set-complement enum-set)
Returns a newly allocated enumeration set that is the complement of enum-set. This procedure is also in R6RS.
(enum-set-projection enum-set enum-type)
Returns a newly allocated enumeration set of type enum-type. Its elements are the enum objects in enum-type which have the same names as members of enum-set. Enum objects without corresponding names are ignored. This procedure is also in R6RS, but uses a second enum-set in place of enum-type.
There will probably be more, depending on how integer sets turn out.
Based on vectors indexed by the enum's ordinal. API is a reduced form of HashTablesCowan.