diff --git a/commit.h b/commit.h
index 6d76959..99a3fea 100644
--- a/commit.h
+++ b/commit.h
@@ -313,7 +313,23 @@ extern int interactive_add(int argc, const char **argv, const char *prefix, int
 extern int run_add_interactive(const char *revision, const char *patch_mode,
 			       const struct pathspec *pathspec);
 
-struct commit_list *reduce_heads(struct commit_list *heads);
+/*
+ * Takes a list of commits and returns a new list where those
+ * have been removed that can be reached from other commits in
+ * the list. It is useful for, e.g., reducing the commits
+ * randomly thrown at the git-merge command and removing
+ * redundant commits that the user shouldn't have given to it.
+ *
+ * This function destroys the STALE bit of the commit objects'
+ * flags.
+ */
+extern struct commit_list *reduce_heads(struct commit_list *heads);
+
+/*
+ * Like `reduce_heads()`, except it replaces the list. Use this
+ * instead of `foo = reduce_heads(foo);` to avoid memory leaks.
+ */
+extern void reduce_heads_replace(struct commit_list **heads);
 
 struct commit_extra_header {
 	struct commit_extra_header *next;