/* delayline.c */
    static double D[M];           // initialized to zero
    static long ptr=0;            // read-write offset

    double delayline(double x)
    {
      double y = D[ptr];          // read operation 
      D[ptr++] = x;               // write operation
      if (ptr >= M) { ptr -= M; } // wrap ptr if needed
//    ptr %= M;                   // modulo-operator syntax
      return y;
    }