diff --git a/ex-2_18-xx.scm b/ex-2_18-32.scm
similarity index 71%
rename from ex-2_18-xx.scm
rename to ex-2_18-32.scm
index 07aa218..8c55ddd 100644
--- a/ex-2_18-xx.scm
+++ b/ex-2_18-32.scm
@@ -1,3 +1,5 @@
+(load "util.scm")
+
 (display "\nex-2.24\n")
 
 (define x (cons (list 1 2) (list 3 4)))
@@ -152,5 +154,53 @@
 (define (make-branch length structure)
   (cons length structure))
 
-(display "\nex-2.30")
+(display "\nex-2.30 - tree square\n")
+
+; using tail recursion
+(define (square-tree t)
+  (cond
+    ((null? t) nil)
+    ((not (pair? t)) (square t))
+    (else (cons (square-tree (car t)) (square-tree (cdr t))))))
+
+(display x) (newline)
+(display (square-tree x)) (newline)
+
+; using map
+(define (square-tree t)
+  (cond
+    ((null? t) nil)
+    ((not (pair? t)) (square t))
+    (else (map square-tree t))))
+
+(display (square-tree x)) (newline)
+
+(display "\nex-2.31\n")
+
+(define (tree-map proc tree)
+  (cond
+    ((null? tree) tree)
+    ((not (pair? tree)) (proc tree))
+    (else (map (lambda (t) tree-map proc t) tree))))
+
+(define (square-tree tree) (tree-map square tree))
+(display (square-tree x)) (newline)
+
+(display "\nex-2.32\n")
+
+(define (subsets s)
+  (if (null? s)
+      (list nil)
+      (let ((rest (subsets (cdr s))))
+        (append rest (map (lambda (r) (cons (car s) r)) rest)))))
+
+; Assuming we have an oracle procedure subsets and we get a new list that we
+; split into car and cdr). If we use the oracle to compute the subsets for cdr
+; then we get a new list rest. To compute the new subsets from that list we
+; have to a) keep the rest as it is and b) add the current element (car) to all
+; the subsets (rest). The only other tricky part (that was already given) is
+; that when we get an empty list we want to return a list including that empty
+; list.
+
+(display (subsets (list 1 2 3)))
 
diff --git a/ex-2_33-xx.scm b/ex-2_33-xx.scm
new file mode 100644
index 0000000..5e2ad75
--- /dev/null
+++ b/ex-2_33-xx.scm
@@ -0,0 +1,8 @@
+(load "util.scm")
+
+
+(display "\nex-2.34\n")
+
+(display "\nex-2.35\n")
+
+(display "\nex-2.36\n")