import std.stdio;
import std.functional;

alias ackerman = memoize!ack;
int ack (int m, int n) {
  if (m == 0) {
    return n + 1;
  }
  else if (n == 0) {
    return ackerman(m - 1, 1);
  }
  return ackerman(m - 1, ackerman(m, n - 1));
}

void main() {
  int i = 4, j = 2;
  writefln("ack(%d, %d) = %d", i, j, ackerman(i, j));
}