public static void main(String[] args) {

          // create two guitar strings, for concert A and C
          double CONCERT_A = 440.0;
          double CONCERT_C = CONCERT_A * Math.pow(1.05956, 3.0);  
          GuitarString stringA = new GuitarString(CONCERT_A);
     //     GuitarString stringC = new GuitarString(CONCERT_C);
          GuitarString stringC = new GuitarString(493.883);

          while (true) {

              // check if the user has typed a key; if so, process it   
              if (StdDraw.hasNextKeyTyped()) {
                  char key = StdDraw.nextKeyTyped();
                  if      (key == 'a') { stringA.pluck(); }
                  else if (key == 'c') { stringC.pluck(); }
              }

              // compute the superposition of samples
              double sample = stringA.sample() + stringC.sample();
  
              // play the sample on standard audio
              StdAudio.play(sample);
  
              // advance the simulation of each guitar string by one step   
              stringA.tic();
              stringC.tic();
          }
       }