diff --git a/.travis.yml b/.travis.yml
index 455f3c0..45c29f6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 language: clojure
-lein: lein2
-script: lein2 midje :config .midje-grading-config.clj
+lein: lein
+script: lein midje :config .midje-grading-config.clj
 jdk:
   - openjdk7
 notifications:
diff --git a/src/recursion.clj b/src/recursion.clj
index 95d77aa..729a43c 100644
--- a/src/recursion.clj
+++ b/src/recursion.clj
@@ -1,92 +1,268 @@
 (ns recursion)
 
 (defn product [coll]
-  :-)
+  (if (empty? coll)
+    1
+    (if (zero? (first coll))
+      0
+      (* (first coll) (product (rest coll))))))
+
+(defn product2 [coll]
+  (apply * coll))
 
 (defn singleton? [coll]
-  :-)
+  (if (or (empty? coll) (not (empty? (rest coll))))
+    false
+    true))
 
 (defn my-last [coll]
-  :-)
+  (if (or (singleton? coll) (empty? coll))
+    (first coll)
+    (my-last (rest coll))))
 
 (defn max-element [a-seq]
-  :-)
+  (if (empty? a-seq)
+    nil
+    (if (singleton? a-seq)
+      (first a-seq)
+      (max (first a-seq) (max-element (rest a-seq))))))
+
+(defn max-element-2 [a-seq]
+  (if (empty? a-seq)
+    nil
+    (apply max a-seq)))
 
 (defn seq-max [seq-1 seq-2]
-  [:-])
+  (let [count-1 (count seq-1)
+        count-2 (count seq-2)]
+    (if (> count-1 count-2) seq-1 seq-2)))
 
 (defn longest-sequence [a-seq]
-  [:-])
+  (if (empty? a-seq)
+    nil
+    (seq-max (first a-seq) (longest-sequence (rest a-seq)))))
 
 (defn my-filter [pred? a-seq]
-  [:-])
+  (if (empty? a-seq)
+    a-seq
+    (if (pred? (first a-seq))
+      (cons (first a-seq) (my-filter pred? (rest a-seq)))
+      (my-filter pred? (rest a-seq)))))
 
 (defn sequence-contains? [elem a-seq]
-  :-)
+  (cond
+    (empty? a-seq) false
+    (= elem (first a-seq)) true
+    :else (sequence-contains? elem (rest a-seq))))
 
 (defn my-take-while [pred? a-seq]
-  [:-])
+  (cond
+    (empty? a-seq)
+      '()
+    (pred? (first a-seq))
+      (cons (first a-seq) (my-take-while pred? (rest a-seq)))
+    :else
+      '()))
 
 (defn my-drop-while [pred? a-seq]
-  [:-])
+  (cond
+    (empty? a-seq)
+      '()
+    (pred? (first a-seq))
+      (my-drop-while pred? (rest a-seq))
+    :else
+      a-seq))
 
 (defn seq= [a-seq b-seq]
-  :-)
+  (cond
+    (and (empty? a-seq) (empty? b-seq)) true
+    (not= (count a-seq) (count b-seq)) false
+    (not= (first a-seq) (first b-seq))false
+    :else (seq= (rest a-seq) (rest b-seq))))
 
 (defn my-map [f seq-1 seq-2]
-  [:-])
+  (cond
+    (or (empty? seq-1) (empty? seq-2))
+      '()
+    :else
+      (cons
+        (f (first seq-1) (first seq-2))
+        (my-map f (rest seq-1) (rest seq-2)))))
 
 (defn power [n k]
-  :-)
+  (cond
+    (zero? k) 1
+    :else (* n (power n (- k 1)))))
 
 (defn fib [n]
-  :-)
+  (cond
+    (zero? n) 0
+    (= 1 n) 1
+    :else (+ (fib (- n 1)) (fib (- n 2)))))
 
 (defn my-repeat [how-many-times what-to-repeat]
-  [:-])
+  (cond
+    (>= 0 how-many-times) '()
+    :else (cons
+            what-to-repeat
+            (my-repeat (- how-many-times 1) what-to-repeat))))
 
 (defn my-range [up-to]
-  [:-])
+  (cond
+    (>= 0 up-to) `()
+    :else (cons
+            (- up-to 1)
+            (my-range (- up-to 1)))))
 
 (defn tails [a-seq]
-  [:-])
+  (cond
+    (empty? a-seq) `(())
+    :else (cons (sequence a-seq) (tails (rest a-seq)))))
 
 (defn inits [a-seq]
-  [:-])
+  (cond
+    (empty? a-seq) `(())
+    :else (cons (sequence a-seq) (inits (pop a-seq)))))
 
 (defn rotations [a-seq]
-  [:-])
+  (let [t (tails a-seq)
+        i (reverse (inits a-seq))]
+    (cond (empty? a-seq) `(())
+          :else (rest (map concat t i)))))
 
 (defn my-frequencies-helper [freqs a-seq]
-  [:-])
+  (if (empty? a-seq)
+    freqs
+    (let [next-key (first a-seq)
+           new-freq (assoc
+                     freqs
+                     next-key
+                     (inc (get freqs next-key 0)))]
+      (my-frequencies-helper new-freq (rest a-seq)))))
 
 (defn my-frequencies [a-seq]
-  [:-])
+  (my-frequencies-helper {} a-seq))
+
+(defn un-frequencies-helper [a-seq a-map]
+  (if (empty? a-map)
+    a-seq
+    (let [n (last (first a-map))
+          x (first (first a-map))
+          new-seq (concat a-seq (repeat n x))]
+      (un-frequencies-helper new-seq (rest a-map)))))
 
 (defn un-frequencies [a-map]
-  [:-])
+  (un-frequencies-helper [] a-map))
 
+(defn my-take-helper [new-coll n coll]
+  (if (or (zero? n) (empty? coll))
+    new-coll
+    (my-take-helper (conj new-coll (first coll)) (dec n) (rest coll))))
 (defn my-take [n coll]
+  (my-take-helper [] n coll))
+
+(defn my-drop-helper [n coll]
   [:-])
 
 (defn my-drop [n coll]
-  [:-])
+  (if (or (zero? n) (empty? coll))
+    coll
+    (my-drop (dec n) (rest coll))))
 
 (defn halve [a-seq]
-  [:-])
+  (let [n (int (/ (count a-seq) 2))]
+    [(my-take n a-seq) (my-drop n a-seq)]))
+
+(defn seq-merge-helper [final-seq a-seq b-seq]
+  (cond
+    (empty? a-seq) (concat final-seq b-seq)
+    (empty? b-seq) (concat final-seq a-seq)
+    :else (let [a (first a-seq)
+                b (first b-seq)]
+            (cond
+              (> a b) (seq-merge-helper (merge final-seq b) a-seq (rest b-seq))
+              :else (seq-merge-helper (merge final-seq a) (rest a-seq) b-seq)))))
 
 (defn seq-merge [a-seq b-seq]
-  [:-])
+  (seq-merge-helper [] a-seq b-seq))
 
 (defn merge-sort [a-seq]
-  [:-])
+  (let [[x y] (halve a-seq)
+        x-sorted (cond (> 2 (count x)) x :else (merge-sort x))
+        y-sorted (cond (> 2 (count y)) y :else (merge-sort y))]
+    (seq-merge x-sorted y-sorted)))
 
+(defn add-to-last [result x]
+  (conj (vec (drop-last result)) (vec (conj (last result) x))))
+(defn add-to-new [result x]
+  (conj result [x]))
+(defn m-helper [result a-seq]
+  (cond
+    (empty? a-seq) result
+    (or
+      (= 1 (count (last result)))
+      (or
+        (> (first a-seq) (last (last result)) (last (drop-last (last result))))
+        (< (first a-seq) (last (last result)) (last (drop-last (last result))))))
+          (m-helper
+            (add-to-last result (first a-seq))
+            (vec (rest a-seq)))
+    :else (m-helper
+            (add-to-new result (first a-seq))
+            (vec (rest a-seq)))))
 (defn split-into-monotonics [a-seq]
-  [:-])
+  (m-helper [[(first a-seq)]] (vec (rest a-seq))))
+
+; --- permutations ---
+(defn v-insert [vec index x]
+  (concat (take index vec) [x] (drop index vec)))
+
+(defn p-for-one [old x]
+  (for [i (range 0 (inc (count old)))]
+    (v-insert (vec old) i x)))
+
+(defn p-for-many [old x]
+  (apply concat (for [p old] (p-for-one p x))))
+
+(defn p-helper [result index a-set]
+  (cond
+    (= index (count a-set)) result
+    :else (p-helper
+            (p-for-many result (nth (vec a-set) index))
+            (inc index)
+            a-set)))
 
 (defn permutations [a-set]
-  [:-])
+  (p-helper (list '()) 0 a-set))
+
+; --- powerset ---
+(defn exp [x n]
+  (reduce * (repeat n x)))
+
+(defn base2 [x]
+  (Integer/toString x 2))
+
+(defn pad0 [x length]
+  (clojure.string/replace (format (str "%" length "s") x) #" " "0"))
+
+(defn p-set-in-base2 [a-set]
+  (map
+    (fn [x] (pad0 (base2 x) (count a-set)))
+    (range 0 (exp 2 (count a-set)))))
+
+(defn replce-helper [result base2-string a-set]
+  (cond
+    (= 0 (count a-set)) result
+    (= \0 (first base2-string)) (replce-helper result (rest base2-string) (rest a-set))
+    :else (replce-helper (conj result (first a-set)) (rest base2-string) (rest a-set))))
+
+(defn replace [base2-string a-set]
+  (replce-helper #{} base2-string a-set))
 
 (defn powerset [a-set]
-  [:-])
+  (cond
+    (empty? a-set) #{a-set}
+    :else (let [base2-p-set (p-set-in-base2 a-set)]
+            (map #(replace % a-set) base2-p-set))))
+