diff --git a/test_brandt.c b/test_brandt.c
index d18b17a..1540583 100644
--- a/test_brandt.c
+++ b/test_brandt.c
@@ -364,6 +364,18 @@ run_auction (void *arg)
 }
 
 
+/**
+ * Test a specific auction setup.
+ *
+ * @param[in] n number of bidders.
+ * @param[in] k number of different prices.
+ * @param[in] bids The bid array. Must contain exactly @a n elements each less
+ * than @a k.
+ * @param[in] m If 0 it's a first price auction, else an M+1st price auction
+ * specifying the M.
+ * @param[in] outcome_public 1 means public outcome, 0 means private outcome.
+ * @return 0 on success, 1 on failure
+ */
 static int
 test_auction (uint16_t n,
               uint16_t k,