diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index d2d461e..14d008a 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -581,8 +581,10 @@
    log2: log to base 2.
 
    Uses an algorithm that should:
+
      (a) produce exact results for powers of 2, and
-     (b) be monotonic, assuming that the system log is monotonic.
+     (b) give a monotonic log2 (for positive finite floats),
+         assuming that the system log is monotonic.
 */
 
 static double