+typedef struct ccase *sort_distinct_combine_func (struct ccase *first,
+ struct ccase *second,
+ void *aux);
+typedef void sort_distinct_destroy_func (void *aux);
+struct casewriter *sort_distinct_create_writer (const struct subcase *,
+ const struct caseproto *,
+ sort_distinct_combine_func *,
+ sort_distinct_destroy_func *,
+ void *aux);
+struct casereader *sort_distinct_execute (struct casereader *,
+ const struct subcase *,
+ sort_distinct_combine_func *,
+ sort_distinct_destroy_func *,
+ void *aux);
+