anchor.diff:  //
   namespace ExtCore.Collections
   
  +open System.Collections
   open System.Collections.Generic
   open System.Diagnostics
   open LanguagePrimitives
  
positive.code:                 filteredMap
              else
                  filteredMap.Remove key), this)
  
      //
      // OPTIMIZE : We should be able to implement an optimized version
  
negative.code: [<Sealed>]
  [<DebuggerTypeProxy(typedefof<IntMapDebuggerProxy<int>>)>]
  [<DebuggerDisplay("Count = {Count}")>]
  type IntMap< [<EqualityConditionalOn>] 'T> private (trie : PatriciaMap<'T>) =
      /// The empty IntMap.
      static member Empty
          with get () : IntMap<'T> =
  