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diff --git a/src/Data/Array/Nested/Mixed/Shape/Internal.hs b/src/Data/Array/Nested/Mixed/Shape/Internal.hs
new file mode 100644
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+++ b/src/Data/Array/Nested/Mixed/Shape/Internal.hs
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+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TemplateHaskell #-}
+module Data.Array.Nested.Mixed.Shape.Internal where
+
+import Language.Haskell.TH
+
+
+-- | A TH stub function to avoid having to write the same code three times for
+-- the three kinds of shapes.
+ixFromLinearStub :: String
+                 -> TypeQ -> TypeQ
+                 -> PatQ -> (PatQ -> PatQ -> PatQ)
+                 -> ExpQ -> ExpQ
+                 -> ExpQ
+                 -> DecsQ
+ixFromLinearStub fname' ishty ixty zshC consshC ixz ixcons shtolist = do
+  let fname = mkName fname'
+  typesig <- [t| forall i sh. Num i => $ishty sh -> Int -> $ixty sh i |]
+
+  locals <- [d|
+    -- Unfold first iteration of fromLin to do the range check.
+    -- Don't inline this function at first to allow GHC to inline the outer
+    -- function and realise that 'suffixes' is shared. But then later inline it
+    -- anyway, to avoid the function call. Removing the pragma makes GHC
+    -- somehow unable to recognise that 'suffixes' can be shared in a loop.
+    {-# NOINLINE [0] fromLin0 #-}
+    fromLin0 :: Num i => $ishty sh -> [Int] -> Int -> $ixty sh i
+    fromLin0 sh suffixes i =
+        if i < 0 then outrange sh i else
+        case (sh, suffixes) of
+          ($zshC, _) | i > 0 -> outrange sh i
+                     | otherwise -> $ixz
+          ($(consshC (varP (mkName "n")) (varP (mkName "sh'"))), suff : suffs) ->
+            let (q, r) = i `quotRem` suff
+            in if q >= n then outrange sh i else
+               $ixcons (fromIntegral q) (fromLin sh' suffs r)
+          _ -> error "impossible"
+
+    fromLin :: Num i => $ishty sh -> [Int] -> Int -> $ixty sh i
+    fromLin $zshC _ !_ = $ixz
+    fromLin ($(consshC wildP (varP (mkName "sh'")))) (suff : suffs) i =
+      let (q, r) = i `quotRem` suff  -- suff == shrSize sh'
+      in $ixcons (fromIntegral q) (fromLin sh' suffs r)
+    fromLin _ _ _ = error "impossible"
+
+    {-# NOINLINE outrange #-}
+    outrange :: $ishty sh -> Int -> a
+    outrange sh i = error $ fname' ++ ": out of range (" ++ show i ++
+                            " in array of shape " ++ show sh ++ ")" |]
+
+  body <- [|
+    \sh ->  -- give this function arity 1 so that 'suffixes' is shared when
+            -- it's called many times
+      let suffixes = drop 1 (scanr (*) 1 ($shtolist sh))
+      in fromLin0 sh suffixes |]
+
+  return [SigD fname typesig
+         ,FunD fname [Clause [] (NormalB body) locals]]