anchor.diff:  type Key   = bytes * bytes
   type IntKey = cbytes * cbytes
   type Value = bytes * bool
  +type IntValue = cbytes * bool
   
   type dhdb = {
       filename: string;
  
positive.code: (* ------------------------------------------------------------------------------- *)
  let insert self key v =
      let key = DB.serialize<IntKey> (key_to_intkey key) in
    
      let insert (db : DB.db) =
          match DB.get db key with
          | Some _ -> ()
          | None   -> DB.put db key (DB.serialize<Value> v) in
  
      let db = DB.opendb self.filename in
  
  
negative.code: 
      let select (db : DB.db) =
          DB.get db key
              |> Option.map (DB.deserialize<IntValue> >> intvalue_to_value)
            
      let db = DB.opendb self.filename in
  
  