diff --git a/src/Pure/General/set.ML b/src/Pure/General/set.ML
--- a/src/Pure/General/set.ML
+++ b/src/Pure/General/set.ML
@@ -1,481 +1,481 @@
 (*  Title:      Pure/General/set.ML
     Author:     Makarius
 
 Efficient representation of sets (see also Pure/General/table.ML).
 *)
 
 signature SET =
 sig
   structure Key: KEY
   type elem
   type T
   val size: T -> int
   val empty: T
   val build: (T -> T) -> T
   val is_empty: T -> bool
   val fold: (elem -> 'a -> 'a) -> T -> 'a -> 'a
   val fold_rev: (elem -> 'a -> 'a) -> T -> 'a -> 'a
   val dest: T -> elem list
   val min: T -> elem option
   val max: T -> elem option
   val exists: (elem -> bool) -> T -> bool
   val forall: (elem -> bool) -> T -> bool
   val get_first: (elem -> 'a option) -> T -> 'a option
   val member: T -> elem -> bool
   val subset: T * T -> bool
   val eq_set: T * T -> bool
   val insert: elem -> T -> T
   val make: elem list -> T
   val merge: T * T -> T
   val merges: T list -> T
   val remove: elem -> T -> T
   val subtract: T -> T -> T
 end;
 
 functor Set(Key: KEY): SET =
 struct
 
 (* keys *)
 
 structure Key = Key;
 type elem = Key.key;
 
 
 (* datatype *)
 
 datatype T =
   Empty |
   Leaf1 of elem |
   Leaf2 of elem * elem |
   Leaf3 of elem * elem * elem |
   Branch2 of T * elem * T |
   Branch3 of T * elem * T * elem * T |
   Size of int * T;
 
 fun make2 (Empty, e, Empty) = Leaf1 e
   | make2 (Branch2 (Empty, e1, Empty), e2, right) = make2 (Leaf1 e1, e2, right)
   | make2 (left, e1, Branch2 (Empty, e2, Empty)) = make2 (left, e1, Leaf1 e2)
   | make2 (Branch3 (Empty, e1, Empty, e2, Empty), e3, right) = make2 (Leaf2 (e1, e2), e3, right)
   | make2 (left, e1, Branch3 (Empty, e2, Empty, e3, Empty)) = make2 (left, e1, Leaf2 (e2, e3))
   | make2 (Leaf1 e1, e2, Empty) = Leaf2 (e1, e2)
   | make2 (Empty, e1, Leaf1 e2) = Leaf2 (e1, e2)
   | make2 (Leaf1 e1, e2, Leaf1 e3) = Leaf3 (e1, e2, e3)
   | make2 (Leaf2 (e1, e2), e3, Empty) = Leaf3 (e1, e2, e3)
   | make2 (Empty, e1, Leaf2 (e2, e3)) = Leaf3 (e1, e2, e3)
   | make2 arg = Branch2 arg;
 
 fun make3 (Empty, e1, Empty, e2, Empty) = Leaf2 (e1, e2)
   | make3 (Branch2 (Empty, e1, Empty), e2, mid, e3, right) = make3 (Leaf1 e1, e2, mid, e3, right)
   | make3 (left, e1, Branch2 (Empty, e2, Empty), e3, right) = make3 (left, e1, Leaf1 e2, e3, right)
   | make3 (left, e1, mid, e2, Branch2 (Empty, e3, Empty)) = make3 (left, e1, mid, e2, Leaf1 e3)
   | make3 (Leaf1 e1, e2, Empty, e3, Empty) = Leaf3 (e1, e2, e3)
   | make3 (Empty, e1, Leaf1 e2, e3, Empty) = Leaf3 (e1, e2, e3)
   | make3 (Empty, e1, Empty, e2, Leaf1 e3) = Leaf3 (e1, e2, e3)
   | make3 arg = Branch3 arg;
 
 fun unmake (Leaf1 e) = Branch2 (Empty, e, Empty)
   | unmake (Leaf2 (e1, e2)) = Branch3 (Empty, e1, Empty, e2, Empty)
   | unmake (Leaf3 (e1, e2, e3)) =
       Branch2 (Branch2 (Empty, e1, Empty), e2, Branch2 (Empty, e3, Empty))
   | unmake (Size (_, arg)) = arg
   | unmake arg = arg;
 
 
 (* size *)
 
 (*literal copy from table.ML*)
 local
   fun count Empty n = n
     | count (Leaf1 _) n = n + 1
     | count (Leaf2 _) n = n + 2
     | count (Leaf3 _) n = n + 3
     | count (Branch2 (left, _, right)) n = count right (count left (n + 1))
     | count (Branch3 (left, _, mid, _, right)) n = count right (count mid (count left (n + 2)))
     | count (Size (m, _)) n = m + n;
 
   fun box (Branch2 _) = 1
     | box (Branch3 _) = 1
     | box _ = 0;
 
   fun bound arg b =
     if b > 0 then
       (case arg of
         Branch2 (left, _, right) =>
           bound right (bound left (b - box left - box right))
       | Branch3 (left, _, mid, _, right) =>
           bound right (bound mid (bound left (b - box left - box mid - box right)))
       | _ => b)
     else b;
 in
   fun size arg = count arg 0;
   fun make_size m arg = if bound arg 3 <= 0 then Size (m, arg) else arg;
 end;
 
 
 (* empty *)
 
 val empty = Empty;
 
 fun build (f: T -> T) = f empty;
 
 (*literal copy from table.ML*)
 fun is_empty Empty = true
   | is_empty (Size (_, arg)) = is_empty arg
   | is_empty _ = false;
 
 
 (* fold combinators *)
 
 fun fold_set f =
   let
     fun fold Empty a = a
       | fold (Leaf1 e) a = f e a
       | fold (Leaf2 (e1, e2)) a = f e2 (f e1 a)
       | fold (Leaf3 (e1, e2, e3)) a = f e3 (f e2 (f e1 a))
       | fold (Branch2 (left, e, right)) a =
           fold right (f e (fold left a))
       | fold (Branch3 (left, e1, mid, e2, right)) a =
           fold right (f e2 (fold mid (f e1 (fold left a))))
       | fold (Size (_, arg)) a = fold arg a;
   in fold end;
 
 fun fold_rev_set f =
   let
     fun fold_rev Empty a = a
       | fold_rev (Leaf1 e) a = f e a
       | fold_rev (Leaf2 (e1, e2)) a = f e1 (f e2 a)
       | fold_rev (Leaf3 (e1, e2, e3)) a = f e1 (f e2 (f e3 a))
       | fold_rev (Branch2 (left, e, right)) a =
           fold_rev left (f e (fold_rev right a))
       | fold_rev (Branch3 (left, e1, mid, e2, right)) a =
           fold_rev left (f e1 (fold_rev mid (f e2 (fold_rev right a))))
       | fold_rev (Size (_, arg)) a = fold_rev arg a;
   in fold_rev end;
 
 val dest = Library.build o fold_rev_set cons;
 
 
 (* min/max entries *)
 
 fun min Empty = NONE
   | min (Leaf1 e) = SOME e
   | min (Leaf2 (e, _)) = SOME e
   | min (Leaf3 (e, _, _)) = SOME e
   | min (Branch2 (Empty, e, _)) = SOME e
   | min (Branch3 (Empty, e, _, _, _)) = SOME e
   | min (Branch2 (left, _, _)) = min left
   | min (Branch3 (left, _, _, _, _)) = min left
   | min (Size (_, arg)) = min arg;
 
 fun max Empty = NONE
   | max (Leaf1 e) = SOME e
   | max (Leaf2 (_, e)) = SOME e
   | max (Leaf3 (_, _, e)) = SOME e
   | max (Branch2 (_, e, Empty)) = SOME e
   | max (Branch3 (_, _, _, e, Empty)) = SOME e
   | max (Branch2 (_, _, right)) = max right
   | max (Branch3 (_, _, _, _, right)) = max right
   | max (Size (_, arg)) = max arg;
 
 
 (* exists and forall *)
 
 fun exists pred =
   let
     fun ex Empty = false
       | ex (Leaf1 e) = pred e
       | ex (Leaf2 (e1, e2)) = pred e1 orelse pred e2
       | ex (Leaf3 (e1, e2, e3)) = pred e1 orelse pred e2 orelse pred e3
       | ex (Branch2 (left, e, right)) =
           ex left orelse pred e orelse ex right
       | ex (Branch3 (left, e1, mid, e2, right)) =
           ex left orelse pred e1 orelse ex mid orelse pred e2 orelse ex right
       | ex (Size (_, arg)) = ex arg;
   in ex end;
 
 fun forall pred = not o exists (not o pred);
 
 
 (* get_first *)
 
 fun get_first f =
   let
     fun get Empty = NONE
       | get (Leaf1 e) = f e
       | get (Leaf2 (e1, e2)) =
           (case f e1 of
             NONE => f e2
           | some => some)
       | get (Leaf3 (e1, e2, e3)) =
           (case f e1 of
             NONE =>
               (case f e2 of
                 NONE => f e3
               | some => some)
           | some => some)
       | get (Branch2 (left, e, right)) =
           (case get left of
             NONE =>
               (case f e of
                 NONE => get right
               | some => some)
           | some => some)
       | get (Branch3 (left, e1, mid, e2, right)) =
           (case get left of
             NONE =>
               (case f e1 of
                 NONE =>
                   (case get mid of
                     NONE =>
                       (case f e2 of
                         NONE => get right
                       | some => some)
                   | some => some)
               | some => some)
           | some => some)
       | get (Size (_, arg)) = get arg;
   in get end;
 
 
 (* member and subset *)
 
 fun member set elem =
   let
     fun elem_ord e = Key.ord (elem, e)
     val elem_eq = is_equal o elem_ord;
 
     fun mem Empty = false
       | mem (Leaf1 e) = elem_eq e
       | mem (Leaf2 (e1, e2)) =
           (case elem_ord e1 of
             LESS => false
           | EQUAL => true
           | GREATER => elem_eq e2)
       | mem (Leaf3 (e1, e2, e3)) =
           (case elem_ord e2 of
             LESS => elem_eq e1
           | EQUAL => true
           | GREATER => elem_eq e3)
       | mem (Branch2 (left, e, right)) =
           (case elem_ord e of
             LESS => mem left
           | EQUAL => true
           | GREATER => mem right)
       | mem (Branch3 (left, e1, mid, e2, right)) =
           (case elem_ord e1 of
             LESS => mem left
           | EQUAL => true
           | GREATER =>
               (case elem_ord e2 of
                 LESS => mem mid
               | EQUAL => true
               | GREATER => mem right))
       | mem (Size (_, arg)) = mem arg;
   in mem set end;
 
 fun subset (set1, set2) = forall (member set2) set1;
 
 fun eq_set (set1, set2) =
   pointer_eq (set1, set2) orelse size set1 = size set2 andalso subset (set1, set2);
 
 
 (* insert *)
 
 datatype growth = Stay of T | Sprout of T * elem * T;
 
 fun insert elem set =
   if member set elem then set
   else
     let
       fun elem_ord e = Key.ord (elem, e);
-      fun elem_less e = elem_ord e = LESS;
+      val elem_less = is_less o elem_ord;
 
       fun ins Empty = Sprout (Empty, elem, Empty)
         | ins (t as Leaf1 _) = ins (unmake t)
         | ins (t as Leaf2 _) = ins (unmake t)
         | ins (t as Leaf3 _) = ins (unmake t)
         | ins (Branch2 (left, e, right)) =
             (case elem_ord e of
               LESS =>
                 (case ins left of
                   Stay left' => Stay (make2 (left', e, right))
                 | Sprout (left1, e', left2) => Stay (make3 (left1, e', left2, e, right)))
             | EQUAL => Stay (make2 (left, e, right))
             | GREATER =>
                 (case ins right of
                   Stay right' => Stay (make2 (left, e, right'))
                 | Sprout (right1, e', right2) =>
                     Stay (make3 (left, e, right1, e', right2))))
         | ins (Branch3 (left, e1, mid, e2, right)) =
             (case elem_ord e1 of
               LESS =>
                 (case ins left of
                   Stay left' => Stay (make3 (left', e1, mid, e2, right))
                 | Sprout (left1, e', left2) =>
                     Sprout (make2 (left1, e', left2), e1, make2 (mid, e2, right)))
             | EQUAL => Stay (make3 (left, e1, mid, e2, right))
             | GREATER =>
                 (case elem_ord e2 of
                   LESS =>
                     (case ins mid of
                       Stay mid' => Stay (make3 (left, e1, mid', e2, right))
                     | Sprout (mid1, e', mid2) =>
                         Sprout (make2 (left, e1, mid1), e', make2 (mid2, e2, right)))
                 | EQUAL => Stay (make3 (left, e1, mid, e2, right))
                 | GREATER =>
                     (case ins right of
                       Stay right' => Stay (make3 (left, e1, mid, e2, right'))
                     | Sprout (right1, e', right2) =>
                         Sprout (make2 (left, e1, mid), e2, make2 (right1, e', right2)))))
         | ins (Size (_, arg)) = ins arg;
     in
       (case set of
         Empty => Leaf1 elem
       | Leaf1 e =>
           if elem_less e
           then Leaf2 (elem, e)
           else Leaf2 (e, elem)
       | Leaf2 (e1, e2) =>
           if elem_less e1
           then Leaf3 (elem, e1, e2)
           else if elem_less e2
           then Leaf3 (e1, elem, e2)
           else Leaf3 (e1, e2, elem)
       | _ =>
           make_size (size set + 1)
             (case ins set of
               Stay set' => set'
             | Sprout br => make2 br))
     end;
 
 fun make elems = build (fold insert elems);
 
 
 (* merge *)
 
 fun merge (set1, set2) =
   if pointer_eq (set1, set2) then set1
   else if is_empty set1 then set2
   else if is_empty set2 then set1
   else if size set1 >= size set2
   then fold_set insert set2 set1
   else fold_set insert set1 set2;
 
 fun merges sets = Library.foldl merge (empty, sets);
 
 
 (* remove *)
 
 local
 
 fun compare NONE _ = LESS
   | compare (SOME e1) e2 = Key.ord (e1, e2);
 
 fun if_equal ord x y = if is_equal ord then x else y;
 
 exception UNDEF of elem;
 
 fun del (SOME k) Empty = raise UNDEF k
   | del NONE Empty = raise Match
   | del NONE (Leaf1 p) = (p, (true, Empty))
   | del NONE (Leaf2 (p, q)) = (p, (false, Leaf1 q))
   | del k (Leaf1 p) =
       (case compare k p of
         EQUAL => (p, (true, Empty))
       | _ => raise UNDEF (the k))
   | del k (Leaf2 (p, q)) =
       (case compare k p of
         EQUAL => (p, (false, Leaf1 q))
       | _ =>
         (case compare k q of
           EQUAL => (q, (false, Leaf1 p))
         | _ => raise UNDEF (the k)))
   | del k (Leaf3 (p, q, r)) = del k (Branch2 (Leaf1 p, q, Leaf1 r))
   | del k (Branch2 (l, p, r)) =
       (case compare k p of
         LESS =>
           (case del k l of
             (p', (false, l')) => (p', (false, make2 (l', p, r)))
           | (p', (true, l')) => (p', case unmake r of
               Branch2 (rl, rp, rr) =>
                 (true, make3 (l', p, rl, rp, rr))
             | Branch3 (rl, rp, rm, rq, rr) => (false, make2
                 (make2 (l', p, rl), rp, make2 (rm, rq, rr)))))
       | ord =>
           (case del (if_equal ord NONE k) r of
             (p', (false, r')) => (p', (false, make2 (l, if_equal ord p' p, r')))
           | (p', (true, r')) => (p', case unmake l of
               Branch2 (ll, lp, lr) =>
                 (true, make3 (ll, lp, lr, if_equal ord p' p, r'))
             | Branch3 (ll, lp, lm, lq, lr) => (false, make2
                 (make2 (ll, lp, lm), lq, make2 (lr, if_equal ord p' p, r'))))))
   | del k (Branch3 (l, p, m, q, r)) =
       (case compare k q of
         LESS =>
           (case compare k p of
             LESS =>
               (case del k l of
                 (p', (false, l')) => (p', (false, make3 (l', p, m, q, r)))
               | (p', (true, l')) => (p', (false, case (unmake m, unmake r) of
                   (Branch2 (ml, mp, mr), Branch2 _) =>
                     make2 (make3 (l', p, ml, mp, mr), q, r)
                 | (Branch3 (ml, mp, mm, mq, mr), _) =>
                     make3 (make2 (l', p, ml), mp, make2 (mm, mq, mr), q, r)
                 | (Branch2 (ml, mp, mr), Branch3 (rl, rp, rm, rq, rr)) =>
                     make3 (make2 (l', p, ml), mp, make2 (mr, q, rl), rp,
                       make2 (rm, rq, rr)))))
           | ord =>
               (case del (if_equal ord NONE k) m of
                 (p', (false, m')) =>
                   (p', (false, make3 (l, if_equal ord p' p, m', q, r)))
               | (p', (true, m')) => (p', (false, case (unmake l, unmake r) of
                   (Branch2 (ll, lp, lr), Branch2 _) =>
                     make2 (make3 (ll, lp, lr, if_equal ord p' p, m'), q, r)
                 | (Branch3 (ll, lp, lm, lq, lr), _) =>
                     make3 (make2 (ll, lp, lm), lq,
                       make2 (lr, if_equal ord p' p, m'), q, r)
                 | (_, Branch3 (rl, rp, rm, rq, rr)) =>
                     make3 (l, if_equal ord p' p, make2 (m', q, rl), rp,
                       make2 (rm, rq, rr))))))
       | ord =>
           (case del (if_equal ord NONE k) r of
             (q', (false, r')) =>
               (q', (false, make3 (l, p, m, if_equal ord q' q, r')))
           | (q', (true, r')) => (q', (false, case (unmake l, unmake m) of
               (Branch2 _, Branch2 (ml, mp, mr)) =>
                 make2 (l, p, make3 (ml, mp, mr, if_equal ord q' q, r'))
             | (_, Branch3 (ml, mp, mm, mq, mr)) =>
                 make3 (l, p, make2 (ml, mp, mm), mq,
                   make2 (mr, if_equal ord q' q, r'))
             | (Branch3 (ll, lp, lm, lq, lr), Branch2 (ml, mp, mr)) =>
                 make3 (make2 (ll, lp, lm), lq, make2 (lr, p, ml), mp,
                   make2 (mr, if_equal ord q' q, r'))))))
   | del k (Size (_, arg)) = del k arg;
 
 in
 
 fun remove elem set =
   if member set elem
   then make_size (size set - 1) (snd (snd (del (SOME elem) set)))
   else set;
 
 val subtract = fold_set remove;
 
 end;
 
 
 (* ML pretty-printing *)
 
 val _ =
   ML_system_pp (fn depth => fn _ => fn set =>
     ML_Pretty.to_polyml
       (ML_Pretty.enum "," "{" "}" (ML_Pretty.from_polyml o ML_system_pretty) (dest set, depth)));
 
 (*final declarations of this structure!*)
 val fold = fold_set;
 val fold_rev = fold_rev_set;
 
 end;
 
 structure Intset = Set(Inttab.Key);
 structure Symset = Set(Symtab.Key);