code: public static Expression rgba(Generator generator, FunctionCall input) {
          if (input.getParameters().size() == 4) {
              return new Color(input.getExpectedIntParam(0),
                               input.getExpectedIntParam(1),
                               input.getExpectedIntParam(2),
                               input.getExpectedFloatParam(3));
          }
          if (input.getParameters().size() == 2) {
              Color color = input.getExpectedColorParam(0);
              float newA = input.getExpectedFloatParam(1);
              return new Color(color.getR(), color.getG(), color.getB(), newA);
          }
          throw new IllegalArgumentException("rgba must be called with either 2 or 4 parameters. Function call: "
                                             + input);
      }
docs: Creates a color from given RGB and alpha values.
  
  @param generator the surrounding generator
  @param input     the function call to evaluate
  @return the result of the evaluation