def delta_r(step):
edges, _ = r_edges(step)<EOL>return (edges[<NUM_LIT:1>:] - edges[:-<NUM_LIT:1>]), None<EOL>
Cells thickness.

    Args:
        step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
            instance.
    Returns:
        tuple of :class:`numpy.array` and None: the thickness of the cells.
        The second element of the tuple is None.