'Return the nth prime number'

def __getitem__(self, n):

    if isinstance(n, slice):
        self.extend_to_no(n.stop)
        return self._list[(n.start - 1):(n.stop - 1):n.step]
    else:
        n = as_int(n)
        self.extend_to_no(n)
        return self._list[(n - 1)]
