diff --git a/src/ex/q040.clj b/src/ex/q040.clj
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index 0000000..871bad5
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+++ b/src/ex/q040.clj
@@ -0,0 +1,8 @@
+;; https://4clojure.oxal.org/#/problem/040
+ 
+(ns ex.q040)
+
+(defn ans
+  "Problem 40, Interpose a Seq. restriction: interpose"
+  [val coll]
+  (pop (reduce #(conj %1 %2 val) [] coll)))
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diff --git a/test/ex/q040_test.clj b/test/ex/q040_test.clj
new file mode 100644
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+++ b/test/ex/q040_test.clj
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+;; https://4clojure.oxal.org/#/problem/040
+ 
+(ns ex.q040-test
+  (:require [clojure.test :as t]
+            [ex.q040 :as sut]))
+
+(t/deftest test-q40
+  (t/testing "Problem 40, Interpose a Seq"
+    (t/is (= [1 0 2 0 3] (sut/ans 0 [1 2 3])))
+    (t/is (= "one, two, three" (apply str (sut/ans ", " ["one" "two" "three"]))))
+    (t/is (= [:a :z :b :z :c :z :d] (sut/ans :z [:a :b :c :d])))))
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