1  #include <stdio.h>
       2  
       3  int acker(int, int);
       4  
       5  int
       6  main(void)
       7  {
       8      int n = acker(3,6);
       9      if (n != 509)
      10  	printf("acker(3,6) = %d != 509\n", n);
      11      return(0);
      12  }
      13  
      14  int
      15  acker(int x,int y)
      16  {
      17      if (x==0)
      18  	return(y+1);
      19      else if (y==0)
      20  	return(acker(x-1,1));
      21      else
      22  	return(acker(x-1, acker(x, y-1)));
      23  }