static ItIt<BigInt> collatzSequence(long n) {
  ret collatzSequence(bigInt(n));
}

static ItIt<BigInt> collatzSequence(BigInt _n) {
  ret new ItIt<BigInt> {
    BigInt three = bigInt(3);
    BigInt n;
    
    public bool hasNext() {
      ret neq(n, BigInt.ONE);
    }
    
    public BigInt next() {
      if (n == null) ret n = _n; // first number
      if (even(n.intValue()))
        n = div(n, BigInt.TWO);
      else
        n = plus(mul(n, three), BigInt.ONE);
      ret n;
    }
  };
}

Began life as a copy of #1030359