def emtpy_cols(cols, rows, plas, model):
    """
    Helper function for shrink_dict.
    Iterates over the given model and removes all numbers of the coordinates
    from the items in the model. this way there will be remaining lists of
    empty cols, rows and plas. Returns a list of the 3 lists.
    """
    for (x_co, y_co, z_co) in model.keys():
        # remove all coordinate components from the corresponding list.
        if x_co in cols:
            cols.remove(x_co)
        if y_co in rows:
            rows.remove(y_co)
        if z_co in plas:
            plas.remove(z_co)
    #print("empty cols returns cols, rows, plas:", cols, rows, plas)
    return [cols, rows, plas]