diff --git a/src/Cryptol/Symbolic/Prims.hs b/src/Cryptol/Symbolic/Prims.hs
index 5ef9c62a..597a6e80 100644
--- a/src/Cryptol/Symbolic/Prims.hs
+++ b/src/Cryptol/Symbolic/Prims.hs
@@ -9,6 +9,7 @@
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE PatternGuards #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE ViewPatterns #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeSynonymInstances #-}
@@ -19,7 +20,7 @@ import Data.List (genericDrop, genericReplicate, genericSplitAt, genericTake, so
 import Data.Ord (comparing)
 
 import Cryptol.Eval.Monad (Eval)
-import Cryptol.Eval.Value (BitWord(..), EvalPrims(..), enumerateSeqMap)
+import Cryptol.Eval.Value (BitWord(..), EvalPrims(..), enumerateSeqMap, SeqMap(..))
 import Cryptol.Prims.Eval (binary, unary, tlamN, arithUnary,
                            arithBinary, Binary, BinArith,
                            logicBinary, logicUnary, zeroV)
@@ -51,7 +52,9 @@ instance EvalPrims SBool SWord where
   evalPrim Decl { .. } =
       panic "Eval" [ "Unimplemented primitive", show dName ]
 
-  iteValue b x y = iteSValue b <$> x <*> y
+  iteValue b x y
+    | Just b' <- SBV.svAsBool b = if b' then x else y
+    | otherwise = iteSValue b <$> x <*> y
 
 -- See also Cryptol.Prims.Eval.primTable
 primTable :: Map.Map Ident Value
@@ -79,43 +82,54 @@ primTable  = Map.fromList $ map (\(n, v) -> (mkIdent (T.pack n), v))
   , ("^"           , binary (logicBinary SBV.svXOr SBV.svXOr))
   , ("complement"  , unary (logicUnary SBV.svNot SBV.svNot))
   , ("zero"        , tlam zeroV)
-  ]
-{-
   , ("<<"          ,  -- {m,n,a} (fin n) => [m] a -> [n] -> [m] a
       tlam $ \m ->
-      tlam $ \_ ->
+      tlam $ \n ->
       tlam $ \a ->
-      VFun $ \xs ->
+      VFun $ \xs -> return $
       VFun $ \y ->
-        case xs of
-          VWord x -> VWord (SBV.svShiftLeft x (fromVWord y))
-          _ ->
+        xs >>= \case
+          VWord x -> VWord . SBV.svShiftLeft x <$> (fromVWord "<<" =<< y)
+          x -> do
+            x' <- fromSeq x
+            let Nat len = numTValue n
             let shl :: Integer -> Value
-                shl i =
+                shl =
                   case numTValue m of
-                    Inf               -> dropV i xs
-                    Nat j | i >= j    -> replicateV j a (zeroV a)
-                          | otherwise -> catV (dropV i xs) (replicateV i a (zeroV a))
-
-             in selectV shl y)
-
+                    Inf   -> \i -> VStream $ SeqMap $ \idx -> lookupSeqMap x' (i+idx)
+                    Nat j -> \i -> VSeq j (isTBit a) $ SeqMap $ \idx ->
+                                       if i+idx >= j then
+                                         return $ zeroV a
+                                       else
+                                         lookupSeqMap x' (i+idx)
+             in selectV len shl =<< y)
   , (">>"          , -- {m,n,a} (fin n) => [m] a -> [n] -> [m] a
       tlam $ \m ->
-      tlam $ \_ ->
+      tlam $ \n ->
       tlam $ \a ->
-      VFun $ \xs ->
+      VFun $ \xs -> return $
       VFun $ \y ->
-        case xs of
-          VWord x -> VWord (SBV.svShiftRight x (fromVWord y))
-          _ ->
-           let shr :: Integer -> Value
-               shr i =
-                 case numTValue m of
-                   Inf               -> catV (replicateV i a (zeroV a)) xs
-                   Nat j | i >= j    -> replicateV j a (zeroV a)
-                         | otherwise -> catV (replicateV i a (zeroV a)) (takeV (j - i) xs)
-             in selectV shr y)
-
+        xs >>= \case
+          VWord x -> VWord . SBV.svShiftLeft x <$> (fromVWord "<<" =<< y)
+          x -> do
+            x' <- fromSeq x
+            let Nat len = numTValue n
+            let shr :: Integer -> Value
+                shr =
+                  case numTValue m of
+                    Inf   -> \i -> VStream $ SeqMap $ \idx ->
+                                       if idx-i < 0 then
+                                         return $ zeroV a
+                                       else
+                                         lookupSeqMap x' (idx-i)
+                    Nat j -> \i -> VSeq j (isTBit a) $ SeqMap $ \idx ->
+                                       if idx-i < 0 then
+                                         return $ zeroV a
+                                       else
+                                         lookupSeqMap x' (idx-i)
+             in selectV len shr =<< y)
+  ]
+{-
   , ("<<<"         , -- {m,n,a} (fin m, fin n) => [m] a -> [n] -> [m] a
       tlam $ \m ->
       tlam $ \_ ->
@@ -279,19 +293,23 @@ primTable  = Map.fromList $ map (\(n, v) -> (mkIdent (T.pack n), v))
   , ("random"      , panic "Cryptol.Symbolic.Prims.evalECon"
                        [ "can't symbolically evaluae ECRandom" ])
   ]
+-}
 
+selectV :: Integer -> (Integer -> Value) -> Value -> Eval Value
+selectV _len f (VWord v) | Just idx <- SBV.svAsInteger v = return $ f idx
 
-selectV :: (Integer -> Value) -> Value -> Value
-selectV f v = sel 0 bits
+selectV len f v = sel 0 =<< bits
   where
-    bits = map fromVBit (fromSeq v) -- index bits in big-endian order
+    bits = enumerateSeqMap len <$> fromSeq v -- index bits in big-endian order
 
-    sel :: Integer -> [SBool] -> Value
-    sel offset []       = f offset
-    sel offset (b : bs) = iteValue b m1 m2
+    sel :: Integer -> [Eval Value] -> Eval Value
+    sel offset []       = return $ f offset
+    sel offset (b : bs) = do b' <- fromVBit <$> b
+                             iteValue b' m1 m2
       where m1 = sel (offset + 2 ^ length bs) bs
             m2 = sel offset bs
 
+{-
 asWordList :: [Value] -> Maybe [SWord]
 asWordList = go id
  where go :: ([SWord] -> [SWord]) -> [Value] -> Maybe [SWord]