functor TREEDICT( structure T : BTreeSig
                    type Key
                    val < : Key * Key -> bool
                    type Item ) : DictSig =
let structure TS = 
    TREESET(structure T = T
            type Item = Key * Item
            val op < = fn ((k, _), (k', _)) => k < k' )
in
struct
open T
exception Lookup

type Dict = (Key * Item) Tree
type Key  = Key
type Item = Item

val empty = TS.empty
val enter = TS.insert

fun lookup  Lf                   k = raise Lookup
  | lookup  (Nd(lt, (k',e), rt)) k =
      if k  < k' then lookup lt k
      else
      if k' < k  then lookup rt k
      else e ;

fun remove(k, d) = let val e = lookup d k in
                       TS.delete((k,e), d)
                   end handle Lookup => d
end
end