def koreni(lists):
    out = []
    for list in lists:
        # posebni primeri
        if len(list) == 0:
            out.append(list)
            continue
        if len(list[0]) == 0:
            if sum(map(lambda s: len(s), list)) == 0:
                out.append(list)
            continue

        fl = list[0][0].lower()
        if len(list) == len(filter(lambda s: s.lower().startswith(fl), list)):
            out.append(list)
            continue

    return out

print(koreni([['bavto', 'Bcbeceda'],['avto', 'abeceda', ''],['o'],['td', 'Ts'], ['', 's'], ['','',''], []]))

print(koreni([]))