Implement the modulo operation using iteration by while. The modulo operation \(a \bmod b\) returns the remainder of the division of \(a\) by \(b\). You can assume that \(a \geq 0\) and \(b > 0\).

```python
def modulo(a, b):
    """Return a mod b (a>=0, b>0).

    >>> modulo(10, 3)
    1
    >>> modulo(14, 5)
    4
    >>> modulo(9, 9)
    0
    """
```