<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def combine_mv_and_lv(mv, lv):
 """Combine MV and LV grid topology in PyPSA format """


    combined = {
        c: pd.concat([mv[c], lv[c]], axis=0) for c in list(lv.keys())
    }

    combined['Transformer'] = mv['Transformer']

    return combined