fun isEven(n) {
  if (n == 0) {
    return true;
  }
  return isOdd(n - 1);
}

fun isOdd(n) {
  if (n == 0) {
    return false;
  }
  return isEven(n - 1);
}

print(isEven(4)); # expect: true
print(isOdd(3)); # expect: true