+-- from the Glasgow Haskell Compiler sources (/Control/Monad/State/Strict.hs)
+
+-- An example from /The Craft of Functional Programming/, Simon
+-- Thompson (<http://www.cs.kent.ac.uk/people/staff/sjt/>),
+-- Addison-Wesley 1999: \"Given an arbitrary tree, transform it to a
+-- tree of integers in which the original elements are replaced by
+-- natural numbers, starting from 0. The same element has to be
+-- replaced by the same number at every occurrence, and when we meet
+-- an as-yet-unvisited element we have to find a \'new\' number to match
+-- it with:\"
+--
+-- > data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show, Eq)
+-- > type Table a = [a]
+--
+-- > numberTree :: Eq a => Tree a -> State (Table a) (Tree Int)
+-- > numberTree Nil = return Nil
+-- > numberTree (Node x t1 t2)
+-- > = do num <- numberNode x
+-- > nt1 <- numberTree t1
+-- > nt2 <- numberTree t2
+-- > return (Node num nt1 nt2)
+-- > where
+-- > numberNode :: Eq a => a -> State (Table a) Int
+-- > numberNode x
+-- > = do table <- get
+-- > (newTable, newPos) <- return (nNode x table)
+-- > put newTable
+-- > return newPos
+-- > nNode:: (Eq a) => a -> Table a -> (Table a, Int)
+-- > nNode x table
+-- > = case (findIndexInList (== x) table) of
+-- > Nothing -> (table ++ [x], length table)
+-- > Just i -> (table, i)
+-- > findIndexInList :: (a -> Bool) -> [a] -> Maybe Int
+-- > findIndexInList = findIndexInListHelp 0
+-- > findIndexInListHelp _ _ [] = Nothing
+-- > findIndexInListHelp count f (h:t)
+-- > = if (f h)
+-- > then Just count
+-- > else findIndexInListHelp (count+1) f t
+--
+-- numTree applies numberTree with an initial state:
+--
+-- > numTree :: (Eq a) => Tree a -> Tree Int
+-- > numTree t = evalState (numberTree t) []
+--
+-- > testTree = Node "Zero" (Node "One" (Node "Two" Nil Nil) (Node "One" (Node "Zero" Nil Nil) Nil)) Nil
+-- > numTree testTree => Node 0 (Node 1 (Node 2 Nil Nil) (Node 1 (Node 0 Nil Nil) Nil)) Nil
+--
+-- sumTree is a little helper function that does not use the State monad:
+--
+-- > sumTree :: (Num a) => Tree a -> a
+-- > sumTree Nil = 0
+-- > sumTree (Node e t1 t2) = e + (sumTree t1) + (sumTree t2)