anchor.diff:                  filteredMap
               else
                   filteredMap.Remove key), this)
  +
  +    (* OPTIMIZE : The methods below should be replaced with optimized implementations where possible. *)    
   
       //
       // OPTIMIZE : We should be able to implement an optimized version
  
positive.code: //
  namespace ExtCore.Collections
  
  open System.Collections.Generic
  open System.Diagnostics
  open LanguagePrimitives
  
negative.code:     member __.ToSeq () =
          trie.ToSeq ()
  
      //
      member this.Choose (chooser : int -> 'T -> 'U option) : IntMap<'U> =
          let chooser = FSharpFunc<_,_,_>.Adapt chooser
  