anchor.diff:      [<Sealed>]
       type internal Map<'Key,'T,'ComparerTag> when 'ComparerTag :> IComparer<'Key>( comparer: IComparer<'Key>, tree: MapTree<'Key,'T>) =
   
  -        static let refresh (m:Map<_,_,'ComparerTag>) t =    Map<_,_,'ComparerTag>(comparer=m.Comparer, tree=t)
  +        static let refresh (m:Map<_,_,'ComparerTag>) t = 
  +            Map<_,_,'ComparerTag>(comparer=m.Comparer, tree=t)
   
           member s.Tree = tree
           member s.Comparer : IComparer<'Key> = comparer
  
positive.code:                 MapNode(k,v,l,r,m+1)
  
          let rebalance t1 k v t2 =
              let t1h = height t1 
              if  height t2 > t1h + 2 then // right is heavier than left 
                  match t2 with 
                  | MapNode(t2k,t2v,t2l,t2r,_) -> 
                     // one of the nodes must have height > height t1 + 1 
  
negative.code:                      mk (mk t1 k v t2l) t2k t2v t2r
                  | _ -> failwith "rebalance"
              else
                  if t1h > t2h + 2 then // left is heavier than right 
                    match t1 with 
                    | MapNode(t1k,t1v,t1l,t1r,_) -> 
                      // one of the nodes must have height > height t2 + 1 
  