solution to 2.29

diff --git a/src/sicp/ex2_29.clj b/src/sicp/ex2_29.clj
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+(ns sicp.ex2_29
+  (:use [clojure.test]))
+
+(defn make-mobile [left right]
+  (list left right))
+
+(defn make-branch [length structure]
+  (list length structure))
+
+;; a.  Write the corresponding selectors left-branch and
+;;     right-branch, which return the branches of a mobile, and
+;;     branch-length and branch-structure, which return the
+;;     components of a branch.
+(defn left-branch [mobile]
+  (first mobile))
+
+(defn right-branch [mobile]
+  (first (rest mobile)))
+
+(defn branch-length [branch]
+  (first branch))
+
+(defn branch-structure [branch]
+  (first (rest branch)))
+
+(deftest test-constructors-selectors
+  (let [m (make-mobile (make-branch 3 8)
+                      (make-branch 9
+                                   (make-mobile (make-branch 2 1)
+                                                (make-branch 1 2))))]
+    (are [x y] [= x y]
+        m '((3 8) (9 ((2 1) (1 2))))
+        (right-branch m) '(9 ((2 1) (1 2)))
+        (left-branch m) '(3 8)
+        (branch-length (left-branch m)) 3
+        (branch-length (right-branch m)) 9
+        (branch-structure (left-branch m)) 8
+        (branch-structure (right-branch m)) '((2 1) (1 2)))))
+
+;; b.  Using your selectors, define a procedure total-weight that
+;;     returns the total weight of a mobile.
+
+(declare total-branch-weight)
+
+(defn total-weight [mobile]
+  (+ (total-branch-weight (right-branch mobile))
+     (total-branch-weight (left-branch mobile))))
+
+(defn total-branch-weight [branch]
+  (let [structure (branch-structure branch)]
+    (cond (number? structure) structure
+         :else (total-weight structure))))
+
+(deftest test-weights
+  (let [m (make-mobile (make-branch 3 8)
+                      (make-branch 9
+                                   (make-mobile (make-branch 2 1)
+                                                (make-branch 1 2))))]
+    (are [x y] [= x y]
+        (total-weight m) 11)))
+
+;; c. A mobile is said to be balanced if the torque applied by its
+;;    top-left branch is equal to that applied by its top-right branch
+;;    (that is, if the length of the left rod multiplied by the weight
+;;    hanging from that rod is equal to the corresponding product for
+;;    the right side) and if each of the submobiles hanging off its
+;;    branches is balanced. Design a predicate that tests whether a
+;;    binary mobile is balanced.
+
+(defn torque [branch]
+  (* (branch-length branch) (total-branch-weight branch)))
+
+(defn mobile? [structure]
+  (seq? structure))
+
+(defn balanced? [mobile]
+  (let [lstructure   (branch-structure (left-branch mobile))
+       rstructure   (branch-structure (right-branch mobile))]
+    (and (= (torque (left-branch mobile))
+           (torque (right-branch mobile)))
+        (if (mobile? lstructure)
+          (balanced? lstructure)
+          true)
+        (if (mobile? rstructure)
+          (balanced? rstructure)
+          true))))
+
+(deftest test-balance
+  (let [m1 (make-mobile (make-branch 3 9)
+                       (make-branch 9
+                                    (make-mobile (make-branch 2 1)
+                                                 (make-branch 1 2))))
+       m2 (make-mobile (make-branch 2 4)
+                       (make-branch 3
+                                    (make-mobile (make-branch 2 1)
+                                                 (make-branch 4 2))))]
+    (are [x y] [= x y]
+        (balanced? m1) true
+        (balanced? m2) false)))
+
+;; d. Suppose we change the representation of mobiles.
+;;    How much do you need to change your programs to convert
+;;    to the new representation?
+
+;; soln
+;; if I were to do it in scheme, then oly selectors would change.
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