anchor.diff:          if m.IsConstant then
               m.GetValue() |> f :> IMod<_>
           else
  -            let f = scoped f
               let inner : ref<Option<'a * IMod<'b>>> = ref None
   
   
  
positive.code:         let scope = Ag.getContext()
          fun v -> Ag.useScope scope (fun () -> f v)
  
      let private scoped2 (f : 'a -> 'b -> 'c) =
          let scope = Ag.getContext()
          fun a b -> Ag.useScope scope (fun () -> f a b)
  
      let private callbackTable = ConditionalWeakTable<IMod, ConcurrentHashSet<IDisposable>>()
      /// <summary>
  
negative.code:     /// cell to be constant in any case.
      /// </summary>
      let custom (compute : unit -> 'a) : IMod<'a> =
          LazyMod(compute) :> IMod<_>
  
      
      /// <summary>
  