def iteration_name(iteration: int) -> str:  # pragma: no cover
    """In each directory we index iteration-specific files by the step number. We pad the number with zeros, so the
    order is lexicographical.

    Raises:
        ValueError, if ``step`` is not between 0 and 99.
    """
    if iteration < 0 or iteration > 99:
        raise ValueError(f"Step {iteration} out of bounds [0, 99].")

    return "iteration-{:02}".format(iteration)