anchor.diff:  /// A bi-directional map.
   [<Sealed>]
   [<NoEquality; NoComparison>]
  -//[<DebuggerTypeProxy(typedefof<BimapDebuggerProxy<int,int>>)>]
  +[<DebuggerTypeProxy(typedefof<BimapDebuggerProxy<int,int>>)>]
   [<DebuggerDisplay("Count = {Count}")>]
   type Bimap<'T1, 'T2
       when 'T1 : comparison
  
positive.code: //
  namespace ExtCore.Collections
  
  open System.Diagnostics
  open LanguagePrimitives
  open OptimizedClosures
  
negative.code:             this
  
      //
      member __.Iterate (action : 'T1 -> 'T2 -> unit) : unit =
          Map.iter action left
  
      //
  