diff --git a/t/math.t b/t/math.t
index dc1e575..0f73f3d 100644
--- a/t/math.t
+++ b/t/math.t
@@ -14,3 +14,17 @@
 
 (expect "max - 2 parameters" (= (max 5 30) 30))
 (expect "min - 2 parameters" (= (min 5 30) 5))
+
+
+(comment (mod -1 5))
+(expect "mod - (mod -1 5) == 4"
+        (= (mod -1 5) 4))
+
+(expect "mod - (mod 13 4) == 1"
+        (= (mod 13 4) 1))
+
+(expect "mod - (mod 13.5 1) == 0.5"
+        (= (mod 13.5 1) 0.5))
+
+(expect "mod - (mod -13.5 1) == 0.5"
+        (= (mod -13.5 1) 0.5))