From f8310270c46eac78e337d178964b98f186266427 Mon Sep 17 00:00:00 2001
From: rsiddharth <s@ricketyspace.net>
Date: Sat, 10 Aug 2019 17:19:06 -0400
Subject: Add (net rickteyspace sicp two thirtyone).

---
 net/ricketyspace/sicp/two/thirtyone.scm | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
 create mode 100644 net/ricketyspace/sicp/two/thirtyone.scm

diff --git a/net/ricketyspace/sicp/two/thirtyone.scm b/net/ricketyspace/sicp/two/thirtyone.scm
new file mode 100644
index 0000000..6420e25
--- /dev/null
+++ b/net/ricketyspace/sicp/two/thirtyone.scm
@@ -0,0 +1,22 @@
+;;;; License: CC0-1.0
+
+(define-module (net ricketyspace sicp two thirtyone)
+  #:export (tree-map
+            square-tree))
+
+(define (tree-map fun tree)
+  (cond ((null? tree) '())
+        ((not (pair? tree)) (fun tree))
+        (else (cons (tree-map fun (car tree))
+                    (tree-map fun (cdr tree))))))
+
+(define (square-tree tree) (tree-map (lambda (x) (* x x)) tree))
+
+;;; Guile REPL
+;;;
+;;; scheme@(guile-user)> ,user (net ricketyspace sicp two thirtyone)
+;;; scheme@(guile-user)> (square-tree
+;;;                        (list 1
+;;;                              (list 2 (list 3 4) 5)
+;;;                              (list 6 7)))
+;;; $2 = (1 (4 (9 16) 25) (36 49))
-- 
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