code: @Override
      public void toFile(File file, Engine engine) throws IOException {
          if (engine.getInputVariables().isEmpty()) {
              throw new RuntimeException("[exporter error] engine has no input variables to export the surface");
          }
          if (engine.getOutputVariables().isEmpty()) {
              throw new RuntimeException("[exporter error] engine has no output variables to export the surface");
          }
          InputVariable a = engine.getInputVariables().get(0);
          InputVariable b = engine.getInputVariables().get(1 % engine.numberOfInputVariables());
          toFile(file, engine, a, b,
                  1024, FldExporter.ScopeOfValues.AllVariables, engine.getOutputVariables());
      }
docs: Creates an R script file plotting multiple surfaces based on a data frame
  generated with 1024 values in the scope of FldExporter::AllVariables for
  the two input variables
  
  @param file is the R script file
  @param engine is the engine to export
  @throws IOException if any error occurs during writing the file