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diff --git a/net/ricketyspace/sicp/one/thirty.scm b/net/ricketyspace/sicp/one/thirty.scm
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+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one thirty)
+  #:export (sum))
+
+(define (sum term a next b)
+  (define (iter a result)
+    (if (> a b)
+        result
+        (iter (next a) (+ (term a) result))))
+  (iter a 0))
+
+;;; GNU Guile REPL
+;;;
+;;; scheme@(guile-user)> ,use (net ricketyspace sicp one thirty)
+;;; scheme@(guile-user)> (define (inc n) (+ n 1))
+;;; scheme@(guile-user)> (define (cube x) (* x x x))
+;;; scheme@(guile-user)> (sum cube 1 inc 10)
+;;; scheme@(guile-user)> (define (pi-sum a b)
+;;;   (define (pi-term x)
+;;;     (/ 1.0 (* x (+ x 2))))
+;;;   (define (pi-next x) (+ x 4))
+;;; (sum pi-term a pi-next b))
+;;; scheme@(guile-user)> (* 8 (pi-sum 1 1000))
+;;; $9 = 3.139592655589782
+;;;