diff --git a/v4/tests/test_foundation.c b/v4/tests/test_foundation.c
index 299d4d8e..4387b674 100644
--- a/v4/tests/test_foundation.c
+++ b/v4/tests/test_foundation.c
@@ -15,8 +15,10 @@
  * built as q*n + r.  D+ is the call-free version in 5.7, checked against C
  * over every quadruple of the edge vectors and pseudo-random pairs.  /MOD,
  * U>, ABS, S>D, DABS, M+, D-, D0= and D= are as written in section 5 and
- * checked against C.  Every one is also probed for how much of
- * the 10- and 9-deep circular stacks (D-2) it leaves to its caller.
+ * checked against C.  Q.+ and Q.- (section 5.26: the D+ and D- words) are
+ * checked against v3's 64-bit wrapping q48_add and q48_sub.  Every word is
+ * also probed for how much of the 10- and 9-deep circular stacks (D-2) it
+ * leaves to its caller.
  *
  * Every call is made with a canary under the arguments, and the canary must
  * still be directly under the results afterwards: a definition that leaves