+/* Creates and returns a new casegrouper that takes its input
+ from READER. SAME_GROUP is used to decide which cases are in
+ a group: it returns true if the pair of cases provided are in
+ the same group, false otherwise. DESTROY will be called when
+ the casegrouper is destroyed and should free any storage
+ needed by SAME_GROUP.
+
+ Takes ownerhip of READER.
+
+ SAME_GROUP may be a null pointer. If so, READER's entire
+ contents is considered to be a single group. */