diff --git a/tapl/mlish.rkt b/tapl/mlish.rkt
index 21a7897..5cc5a0b 100644
--- a/tapl/mlish.rkt
+++ b/tapl/mlish.rkt
@@ -540,6 +540,7 @@
 (define-primop string : (→ Char String))
 (define-primop sleep : (→ Int Unit))
 (define-primop string=? : (→ String String Bool))
+(define-primop string<=? : (→ String String Bool))
 
 (define-typed-syntax string-append
   [(_ . strs)
diff --git a/tapl/tests/mlish/bg/README.md b/tapl/tests/mlish/bg/README.md
index 4a56998..33100ed 100644
--- a/tapl/tests/mlish/bg/README.md
+++ b/tapl/tests/mlish/bg/README.md
@@ -45,4 +45,36 @@ mlish tests by Ben
 (map-cps [f : (→ A B)] [x* : (List A)] → (List B))
 ```
 
+`huffman`
+---
+```
+(empty → Symbol*)
+(singleton [s : String] → Symbol*)
+(insert [s* : Symbol*] [s1 : String] → Symbol*)
+(union [s1 : Symbol*] [s2 : Symbol*] → Symbol*)
+(contains [s* : Symbol*] [s : Symbol] → Bool)
+(list [x : A] → (List A))
+(append [x* : (List A)] [y* : (List A)] → (List A))
+(length [x* : (List A)] → Int)
+(symbols [h : HTree] → Symbol*)
+(weight [h : HTree] → Int)
+(make-code-tree [left : HTree] [right : HTree] → HTree)
+(decode-aux [bits : Bit*] [root : HTree] [current-branch : HTree] → SymbolList)
+(decode [bits : Bit*] [tree : HTree] → SymbolList)
+(choose-branch [bit : Bit] [branch : HTree] → HTree)
+(adjoin-set [x : HTree] [set : HTreeSet] → HTreeSet)
+(make-leaf-set [pair* : (List (× Symbol Int))] → HTreeSet)
+sample-tree
+sample-message
+(encode [message : SymbolList] [tree : HTree] → Bit*)
+(contains-symbol [s : Symbol] [tree : HTree] → Bool)
+(encode-symbol [s : Symbol] [tree : HTree] → Bit*)
+(generate-huffman-tree [pair* : (List (× Symbol Frequency))] → HTree)
+(successive-merge [tree* : HTreeSet] → HTree)
+rock-pair*
+rock-tree (generate-huffman-tree rock-pair*))
+rock-message
+rock-bit* (encode rock-message rock-tree))
+```
+
 
diff --git a/tapl/tests/mlish/bg/huffman.mlish b/tapl/tests/mlish/bg/huffman.mlish
new file mode 100644
index 0000000..0e3399a
--- /dev/null
+++ b/tapl/tests/mlish/bg/huffman.mlish
@@ -0,0 +1,278 @@
+#lang s-exp "../../../mlish.rkt"
+(require "../../rackunit-typechecking.rkt")
+
+;; Huffman trees from SICP
+
+;; =============================================================================
+;; === Sets of Symbols
+
+(define-type-alias Symbol String)
+
+;; Set of strings
+(define-type Symbol*
+  [Empty]
+  [Singleton String]
+  [Join String Symbol* Symbol*])
+
+(define (empty → Symbol*)
+  Empty)
+
+(define (singleton [s : String] → Symbol*)
+  (Singleton s))
+
+(define (insert [s* : Symbol*] [s1 : String] → Symbol*)
+  (match s* with
+   [Empty -> (singleton s1)]
+   [Singleton s2 ->
+    (if (string<=? s1 s2)
+      (if (string=? s1 s2)
+        s*
+        (Join s2 (singleton s1) (empty)))
+      (Join s1 (singleton s2) (empty)))]
+   [Join s2 l* r* ->
+    (if (string<=? s1 s2)
+      (if (string=? s1 s2)
+        s*
+        (Join s2 (insert l* s1) r*))
+      (Join s2 l* (insert r* s1)))]))
+
+(define (union [s1 : Symbol*] [s2 : Symbol*] → Symbol*)
+  (match s1 with
+   [Empty -> s2]
+   [Singleton s -> (insert s2 s)]
+   [Join s l* r* -> (union l* (union r* (insert s2 s)))]))
+
+(define (contains [s* : Symbol*] [s : Symbol] → Bool)
+  (match s* with
+   [Empty -> #f]
+   [Singleton s2 -> (string=? s s2)]
+   [Join s2 l* r* ->
+    (if (string<=? s s2)
+      (if (string=? s s2)
+        #t
+        (contains l* s))
+      (contains r* s))]))
+
+;; -----------------------------------------------------------------------------
+
+(check-type
+  (insert (empty) "hello")
+  : Symbol*
+  ⇒ (singleton "hello"))
+
+(check-type
+  (insert (insert (empty) "a") "b")
+  : Symbol*
+  ⇒ (Join "b" (singleton "a") (empty)))
+
+(check-type
+  (insert (insert (empty) "b") "a")
+  : Symbol*
+  ⇒ (Join "b" (singleton "a") (empty)))
+
+(check-type
+  (insert (insert (insert (empty) "a") "b") "c")
+  : Symbol*
+  ⇒ (Join "b" (singleton "a") (singleton "c")))
+
+(check-type
+  (insert (insert (insert (empty) "c") "b") "a")
+  : Symbol*
+  ⇒ (Join "c" (Join "b" (singleton "a") (empty)) (empty)))
+
+(check-type
+  (union
+    (insert (insert (insert (empty) "c") "b") "a")
+    (insert (insert (insert (empty) "a") "b") "c"))
+  : Symbol*
+  ⇒ (Join "b" (singleton "a") (singleton "c")))
+
+;; -----------------------------------------------------------------------------
+
+(define-type (List A)
+  [⊥]
+  [∷ A (List A)])
+
+(define-type-alias SymbolList (List Symbol))
+
+(define (list [x : A] → (List A))
+  (∷ x ⊥))
+
+(define (append [x* : (List A)] [y* : (List A)] → (List A))
+  (match x* with
+   [⊥ -> y*]
+   [∷ x x* ->
+    (∷ x (append x* y*))]))
+
+(define (length [x* : (List A)] → Int)
+  (match x* with
+   [⊥ -> 0]
+   [∷ x x* -> (+ 1 (length x*))]))
+
+;; -----------------------------------------------------------------------------
+
+(define-type Bit O I)
+(define-type-alias Bit* (List Bit))
+
+;; -----------------------------------------------------------------------------
+
+(define-type HTree
+  [Leaf String Int] ;; Symbol, Weight
+  [Node HTree HTree Symbol* Int] ;; Left, Right, Symbols, Weight
+)
+
+(define (symbols [h : HTree] → Symbol*)
+  (match h with
+   [Leaf s w -> (singleton s)]
+   [Node lh rh s* w -> s*]))
+
+(define (weight [h : HTree] → Int)
+  (match h with
+   [Leaf s w -> w]
+   [Node l r s w -> w]))
+
+(define (make-code-tree [left : HTree] [right : HTree] → HTree)
+  (Node left right
+    (union (symbols left) (symbols right))
+    (+ (weight left) (weight right))))
+
+(define (decode-aux [bits : Bit*] [root : HTree] [current-branch : HTree] → SymbolList)
+  (match bits with
+   [⊥ ->
+    ⊥]
+   [∷ b bit* ->
+    (match (choose-branch b current-branch) with
+     [Leaf s w ->
+      (∷ s (decode-aux bit* root root))]
+     [Node l r s* w ->
+      (decode-aux bit* root (Node l r s* w))])]))
+
+(define (decode [bits : Bit*] [tree : HTree] → SymbolList)
+  (decode-aux bits tree tree))
+
+(define (choose-branch [bit : Bit] [branch : HTree] → HTree)
+  (match branch with
+   [Leaf s w ->
+    ;; Error
+    (Leaf "ERROR" 0)]
+   [Node l* r* s* w ->
+    (match bit with
+     [O -> l*]
+     [I -> r*])]))
+
+(define-type-alias HTreeSet (List HTree))
+
+(define (adjoin-set [x : HTree] [set : HTreeSet] → HTreeSet)
+  (match set with
+   [⊥ -> (list x)]
+   [∷ y y* ->
+    (if (< (weight x) (weight y))
+      (∷ x set)
+      (∷ y (adjoin-set x y*)))]))
+
+(define (make-leaf-set [pair* : (List (× Symbol Int))] → HTreeSet)
+  (match pair* with
+   [⊥ -> ⊥]
+   [∷ pair pair* ->
+    (match pair with
+     [s i ->
+      (adjoin-set (Leaf s i) (make-leaf-set pair*))])]))
+
+(check-type
+  (make-leaf-set (∷ (tup "A" 4)
+                 (∷ (tup "B" 2)
+                 (∷ (tup "C" 1)
+                 (∷ (tup "D" 1)
+                 ⊥)))))
+  : HTreeSet
+  ⇒ (∷ (Leaf "D" 1)
+    (∷ (Leaf "C" 1)
+    (∷ (Leaf "B" 2)
+    (∷ (Leaf "A" 4)
+    ⊥)))))
+
+(define sample-tree
+  (make-code-tree
+    (Leaf "A" 4)
+    (make-code-tree
+      (Leaf "B" 2)
+      (make-code-tree
+        (Leaf "D" 1)
+        (Leaf "C" 1)))))
+
+(define sample-message
+  (∷ O (∷ I  (∷ I  (∷ O  (∷ O  (∷ I  (∷ O  (∷ I  (∷ O  (∷ I  (∷ I  (∷ I  (∷ I  (∷ O ⊥)))))))))))))))
+
+(check-type
+  (decode sample-message sample-tree)
+  : SymbolList
+  ⇒  (∷ "A" (∷ "D" (∷ "A" (∷ "B" (∷ "B" (∷ "C" (∷ "B" ⊥))))))))
+
+(define (encode [message : SymbolList] [tree : HTree] → Bit*)
+  (match message with
+   [⊥ -> ⊥]
+   [∷ m m* ->
+    (append (encode-symbol m tree) (encode m* tree))]))
+
+(define (contains-symbol [s : Symbol] [tree : HTree] → Bool)
+  (contains (symbols tree) s))
+
+;; Undefined if symbol is not in tree. Be careful!
+(define (encode-symbol [s : Symbol] [tree : HTree] → Bit*)
+  (match tree with
+   [Leaf s w -> ⊥]
+   [Node l* r* s* w ->
+    (if (contains-symbol s l*)
+      (∷ O (encode-symbol s l*))
+      (∷ I (encode-symbol s r*)))]))
+
+(check-type
+  (encode (decode sample-message sample-tree) sample-tree)
+  : Bit*
+  ⇒ sample-message)
+
+(define-type-alias Frequency Int)
+(define (generate-huffman-tree [pair* : (List (× Symbol Frequency))] → HTree)
+  (successive-merge (make-leaf-set pair*)))
+
+(define (successive-merge [tree* : HTreeSet] → HTree)
+  (match tree* with
+   [⊥ -> (Leaf "ERROR" 0)]
+   [∷ t t* ->
+    (match t* with
+     [⊥ -> t]
+     [∷ t2 t* ->
+      (successive-merge (adjoin-set (make-code-tree t t2) t*))])]))
+
+(define rock-pair*
+  (∷ (tup "A" 2)
+  (∷ (tup "BOOM" 2)
+  (∷ (tup "GET" 2)
+  (∷ (tup "JOB" 2)
+  (∷ (tup "NA" 16)
+  (∷ (tup "SHA" 3)
+  (∷ (tup "YIP" 9)
+  (∷ (tup "WAH" 1)
+     ⊥)))))))))
+
+(define rock-tree (generate-huffman-tree rock-pair*))
+
+(define rock-message
+  (∷ "GET" (∷ "A" (∷ "JOB"
+  (∷ "SHA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA"
+  (∷ "GET" (∷ "A" (∷ "JOB"
+  (∷ "SHA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA" (∷ "NA"
+  (∷ "WAH" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP" (∷ "YIP"
+  (∷ "SHA" (∷ "BOOM" ⊥)))))))))))))))))))))))))))))))))))))
+
+(define rock-bit* (encode rock-message rock-tree))
+
+(check-type
+  (decode rock-bit* rock-tree)
+  : SymbolList
+  ⇒ rock-message)
+
+(check-type
+  (length rock-bit*)
+  : Int
+  ⇒ 84)