diff --git a/src/libraries/OS-Independent/Data/Set.dcl b/src/libraries/OS-Independent/Data/Set.dcl
index e3373eaeaab576eb0633ba0554186a00aa162f46..70a3a85146b136e80b5edb298088d066ad1b5248 100644
--- a/src/libraries/OS-Independent/Data/Set.dcl
+++ b/src/libraries/OS-Independent/Data/Set.dcl
@@ -82,17 +82,17 @@ from Data.Foldable import class Foldable
  * @invariant integrity: A.s :: Set a | Eq, genShow{|*|}, gPrint{|*|} a:
  *     // Check that the data structure is still correct.
  *     name "no_duplicates" (no_duplicates s) /\
- *     name "log_size"      (log_size s) /\
+ *     name "balanced"      (balanced s) /\
  *     name "sizes_correct" (sizes_correct s)
  *
  * @invariant no_duplicates: A.s :: Set a | Eq, genShow{|*|}, gPrint{|*|} a:
  *     xs =.= removeDup xs where xs = toList s
  *
- * @invariant log_size: A.s :: Set a:
- *     check (<) nelem (2 ^ depth s)
+ * @invariant balanced: A.s :: Set a:
+ *     case s of
+ *         Tip -> prop True
+ *         Bin _ _ l r -> abs (depth l - depth r) < 2 /\ balanced l /\ balanced r
  *     where
- *         nelem = size s
- *
  *         depth :: (Set a) -> Int
  *         depth Tip = 0
  *         depth (Bin _ _ l r) = 1 + (max `on` depth) l r
diff --git a/src/libraries/OS-Independent/Data/Set.icl b/src/libraries/OS-Independent/Data/Set.icl
index c70917e8972bfa2197546f59a31d464d81006dc4..6a692fc4306884ddc15ada19be150cd144a008bc 100644
--- a/src/libraries/OS-Independent/Data/Set.icl
+++ b/src/libraries/OS-Independent/Data/Set.icl
@@ -454,7 +454,7 @@ maxView x = Just (deleteFindMax x)
   However (since we use quickcheck :-) we will stick to strictly balanced 
   trees.
 --------------------------------------------------------------------*/
-delta :== 4
+delta :== 3
 ratio :== 2
 
 // Functions balanceL and balanceR are specialised versions of balance.