code: public static Expression opacify(Generator generator, FunctionCall input) {
          Color color = input.getExpectedColorParam(0);
          float amount = input.getExpectedFloatParam(1);
          return new Color(color.getR(), color.getG(), color.getB(), color.getA() + amount);
      }
docs: Increases the opacity of the given color by the given amount.
  
  @param generator the surrounding generator
  @param input     the function call to evaluate
  @return the result of the evaluation