Fix tests for Data.Set; use balanced invariant instead of log_size (which was always True)

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