--- a/exercises/src/SetPairList.hs	Fri Dec 21 01:44:45 2012 +0100
+++ b/exercises/src/SetPairList.hs	Wed Jan 09 19:45:47 2013 +0100
@@ -1,35 +1,48 @@
--- ?
+module SetPairList(Set, empty, insert, isin, size, union, delete) where
 
 -- Abstraction function
 {-
-    alpha (S x) = undefined
+    alpha :: Set a -> {a}
+    alpha (S []) = {}
+    alpha (S ((a, b):xs)) = {a..b} union (alpha xs)
 
     To show: (10 things)
+    alpha empty =  {}
+    invar s ==> alpha (insert x s) = {x} union alpha s
+    invar s ==> alpha (delete x s) = alpha s \ {x}
+    invar s ==> isin x s = x \in alpha s
+    invar s ==> size s = |alpha s|
+    invar s1, invar s2 ==> alpha (union s1 s2) = (alpha s1) union (alpha s2)
+
+    invar empty
+    invar s ==> invar (insert x s)
+    invar s ==> invar(delete x s)
+    invar s1, invar s2 ==> invar (union s1 s2)
 -}
 
 -- Interface
-newtype Set a = S (...)
+newtype Set a = S (PairList a)
 
 invar :: Integral a => Set a -> Bool
-invar (S s) = undefined
+invar (S s) = invarPL s
 
 empty :: Set a
-empty = undefined
+empty = S emptyPL
 
 insert :: Integral a => a -> Set a -> Set a
-insert n (S s) = undefined
+insert n (S s) = S (insertPL n s)
 
 isin :: Integral a => a -> Set a -> Bool
-isin n (S s) = undefined
+isin n (S s) = isinPL n s
 
 size :: Integral a => Set a -> Integer
-size (S s) = undefined
+size (S s) = sizePL s
 
 union :: Integral a => Set a -> Set a -> Set a
-union (S s1) (S s2) = undefined
+union (S s1) (S s2) = S (unionPL s1 s2)
 
 delete :: Integral a => a -> Set a -> Set a
-delete n (S s) = undefined
+delete n (S s) = S (deletePL n s)
 
 -- Implementation
 type PairList a = [(a, a)]