func testFailure() throws {
       // this works fine
       let extjson1 = "{\"a\" : [{\"$numberInt\": \"10\"}]}".data(using: .utf8)!
       let res1 = try Document(fromJSON: extjson1)
       print(res1)

       // this crashes
       let extjson2 = "{\"x\" : { \"$binary\" : {\"base64\" : \"\", \"subType\" : \"00\"}}}".data(using: .utf8)!
       let res2 = try Document(fromJSON: extjson2)
       print(res2)
   }