anchor.diff:  
           let action = FSharpFunc<_,_,_>.Adapt action
   
  -        // Implement based on the standard Set.fold function.
           let state =
               (state, set)
               ||> Set.fold (fun state el ->
  
positive.code: 
          (), state
  
  
negative.code: 
          let action = FSharpFunc<_,_,_>.Adapt action
  
          let state =
              (set, state)
              ||> Set.foldBack (fun el state ->
  