code: protected static Key<Grid> gridKeyName(String modelName, Frame fr) {
      if (fr == null || fr._key == null) {
        throw new IllegalArgumentException("The frame being grid-searched over must have a Key");
      }
      return Key.make("Grid_" + modelName + "_" + fr._key.toString() + H2O.calcNextUniqueModelId(""));
    }
docs: Defines a key for a new Grid object holding results of grid search.
  
  @return a grid key for a particular modeling class and frame.
  @throws java.lang.IllegalArgumentException if frame is not saved to distributed store.