Enumerate the primes using only unary arithmetic

Primes.java

interface NaturalNumber {}

class Zero implements NaturalNumber {
  @Override
  public String toString() {
    return "0";
  }
}

class Successor implements NaturalNumber {
  final NaturalNumber prev;

  Successor(NaturalNumber prev) {
    this.prev = prev;
  }

  @Override
  public String toString() {
    return String.format("S(%s)", prev);
  }
}

class Primes {
  static boolean isGreater(NaturalNumber a, NaturalNumber b) {
    while (a instanceof Successor && b instanceof Successor) {
      a = ((Successor) a).prev;
      b = ((Successor) b).prev;
    }
    return a instanceof Successor;
  }

  static NaturalNumber difference(NaturalNumber a, NaturalNumber b) {
    if (isGreater(b, a))
      throw new IllegalArgumentException("Second operand was greater than first.");

    while (b instanceof Successor) {
      a = ((Successor) a).prev;
      b = ((Successor) b).prev;
    }
    return a;
  }

  static NaturalNumber remainder(NaturalNumber a, NaturalNumber b) {
    while (!isGreater(b, a))
      a = difference(a, b);
    return a;
  }

  static boolean isDivisible(NaturalNumber a, NaturalNumber b) {
    return remainder(a, b) instanceof Zero;
  }

  static boolean isPrime(NaturalNumber n) {
    if (n instanceof Zero || ((Successor) n).prev instanceof Zero)
      return false;

    NaturalNumber d = new Successor(new Successor(new Zero()));
    while (isGreater(n, d)) {
      if (isDivisible(n, d))
        return false;
      d = new Successor(d);
    }
    return true;
  }

  public static void main(String[] args) {
    for (NaturalNumber n = new Zero();; n = new Successor(n))
      if (isPrime(n))
        System.out.println(n);
  }
}