solution to 2.40. Extension of the text discussions.

diff --git a/src/sicp/ex2_40.clj b/src/sicp/ex2_40.clj
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+++ b/src/sicp/ex2_40.clj
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+(ns sicp.ex2_40
+  (:use [sicp.ch2-2 :only (enumerate-interval accumulate append)]
+        [sicp.ch1-2 :only (prime?)]))
+
+;; Define a procedure unique-pairs that, given an integer n,
+;; generates the sequence of pairs (i,j) with 1< j< i< n.
+;; Use unique-pairs to simplify the definition of prime-sum-pairs
+;; given above.
+(defn unique-pairs [n]
+  (accumulate append
+              nil
+              (map (fn [i]
+                     (map (fn [j] (list i j))
+                          (enumerate-interval 1 (- i 1))))
+                   (enumerate-interval 1 n))))
+
+(defn prime-sum? [pair]
+  (prime? (+ (first pair)
+             (first (rest pair)))))
+
+(defn make-pair-sum [pair]
+  (list (first pair) (first (rest pair)) (+ (first pair)
+                                            (first (rest pair)))))
+
+(defn prime-sum-pairs [n]
+  (map make-pair-sum (filter prime-sum? (unique-pairs n))))
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