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diff --git a/src/math/sort.h b/src/math/sort.h
index 96ac32cc0b..2f1d3899a8 100644
--- a/src/math/sort.h
+++ b/src/math/sort.h
@@ -17,6 +17,21 @@
 #ifndef MATH_SORT_H
 #define MATH_SORT_H 1
 
+/* Support for sorting cases.
+
+   Use sort_create_writer() to sort cases in the most general way:
+
+   - Create a casewriter with sort_create_writer(), specifying the sort
+     criteria.
+   - Write all of the cases to be sorted to the casewriter, e.g. with
+     casewriter_write().
+   - Obtain the sorted results with casewriter_make_reader().
+
+  sort_execute() and sort_execute_1var() are shortcuts for situations where the
+  cases are already available from a casereader.
+
+  All of these functions can efficiently sort data bigger than memory. */
+
 struct subcase;
 struct caseproto;
 struct variable;