diff --git a/doc/src/references/gm-state-transitions.rst b/doc/src/references/gm-state-transitions.rst
index 1b1e148..59df767 100644
--- a/doc/src/references/gm-state-transitions.rst
+++ b/doc/src/references/gm-state-transitions.rst
@@ -406,7 +406,7 @@ Core Transition Rules
 
    .. math::
       \gmrule
-      { \mathtt{CaseJump} \begin{bmatrix} t \to c \end{bmatrix} : i
+      { \mathtt{CaseJump} \begin{bmatrix} t : c \end{bmatrix} : i
       & a : s
       & d
       & h
diff --git a/src/GM.hs b/src/GM.hs
index 381d134..7ed184f 100644
--- a/src/GM.hs
+++ b/src/GM.hs
@@ -144,8 +144,6 @@ isFinal st = null $ st ^. gmCode
 
 step :: GmState -> GmState
 step st = case head (st ^. gmCode) of
-    -- TODO: clean this up. let transition functions use the 'state' parameter
-    -- instead of passing it to them.
     Unwind         -> unwindI        
     PushGlobal n   -> pushGlobalI   n
     PushConstr t n -> pushConstrI t n
@@ -164,8 +162,22 @@ step st = case head (st ^. gmCode) of
     Div            -> divI           
     Split      n   -> splitI        n
     Pack       t n -> packI       t n
+    CaseJump    as -> caseJumpI    as
     where
 
+        caseJumpI :: [(Tag, Code)] -> GmState
+        caseJumpI as = st
+                    & advanceCode
+                    & gmCode %~ (i'++)
+            where
+                h = st ^. gmHeap
+                s = st ^. gmStack
+                NConstr t ss = head s
+                             & hViewUnsafe h
+                i' = fromMaybe
+                    (error $ "unmatched tag: " <> show t)
+                    (lookup t as)
+
         packI :: Tag -> Int -> GmState
         packI t n = st
                   & advanceCode