Keabsahan perbuatan hukum yang dilakukan berdasarkan kuasa direksi perseroan terbatas : studi kasus sengketa pengambilalihan saham PT Sima Bintang Niaga Putusan Mahkamah Agung No 1683/K/PID/2012 = The validity of legal act based on the director s power of attorney in a limited liability company case study PT Sima Bintang Niaga