2 files changed, 57 insertions, 4 deletions
diff --git a/net/ricketyspace/sicp/one/twentyseven.scm b/net/ricketyspace/sicp/one/twentyseven.scm
index cc50dc9..c0e258f 100644
--- a/net/ricketyspace/sicp/one/twentyseven.scm
+++ b/net/ricketyspace/sicp/one/twentyseven.scm
@@ -1,10 +1,8 @@
-;;;; Under Creative Commons Attribution-ShareAlike 4.0
-;;;; International. See
-;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+;;;; License: CC0-1.0
 
 (define-module (net ricketyspace sicp one twentyseven)
   #:use-module (srfi srfi-1)
-  #:export (carmichael-numbers-fool-fermat-test?))
+  #:export (carmichael-numbers-fool-fermat-test? prime?))
 
 (define carmichael-numbers '(561 1105 1729 2465 2821 6601))
 
diff --git a/net/ricketyspace/sicp/two/forty.scm b/net/ricketyspace/sicp/two/forty.scm
new file mode 100644
index 0000000..e7c2851
--- /dev/null
+++ b/net/ricketyspace/sicp/two/forty.scm
@@ -0,0 +1,55 @@
+;;;; License: CC0-1.0
+
+(define-module (net ricketyspace sicp two forty)
+  #:use-module (net ricketyspace sicp one twentyseven)
+  #:export (unique-pairs
+            prime-sum-pairs))
+
+
+(define (accumulate op initial sequence)
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (accumulate op initial (cdr sequence)))))
+
+
+(define (enumerate-interval low high)
+  (if (> low high)
+      '()
+      (cons low (enumerate-interval (+ low 1) high))))
+
+
+(define (prime-sum? pair)
+  (prime? (+ (car pair) (cadr pair))))
+
+
+(define (make-pair-sum pair)
+  (list (car pair) (cadr pair) (+ (car pair) (cadr pair))))
+
+
+(define (flatmap proc seq)
+  (accumulate append '() (map proc seq)))
+
+
+(define (unique-pairs n)
+  (flatmap (lambda (i)
+             (map (lambda (j) (list i j))
+                  (enumerate-interval 1 (- i 1))))
+           (enumerate-interval 1 n)))
+
+
+(define (prime-sum-pairs n)
+  (map make-pair-sum
+       (filter prime-sum?
+               (unique-pairs n))))
+
+
+;;;
+;;; Guile REPL
+;;;
+;;; scheme@(guile-user)> ,re (net ricketyspace sicp two forty)
+;;; scheme@(guile-user)> (unique-pairs 6)
+;;; $8 = ((2 1) (3 1) (3 2) (4 1) (4 2) (4 3) (5 1) (5 2) (5 3) (5 4) (6 1) (6 2) (6 3) (6 4) (6 5))
+;;; scheme@(guile-user)> (prime-sum-pairs 6)
+;;; $9 = ((2 1 3) (3 2 5) (4 1 5) (4 3 7) (5 2 7) (6 1 7) (6 5 11))
+;;;