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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def r_edges(step):
 """Cell border. Args: step (:class:`~stagpy.stagyydata._Step`):
 a step of a StagyyData instance. Returns: tuple of :class:`numpy.array`: the position of the bottom and top walls of the cells. The two elements of the tuple are identical. """

    rbot, rtop = misc.get_rbounds(step)
    centers = step.rprof.loc[:, 'r'].values + rbot
    # assume walls are mid-way between T-nodes
    # could be T-nodes at center between walls
    edges = (centers[:-1] + centers[1:]) / 2
    edges = np.insert(edges, 0, rbot)
    edges = np.append(edges, rtop)
    return edges, edges