Analisa hukum cacat yuridis akta pengikatan jual beli dan kuasa dan implikasi hukum atas gugatan para ahli waris (Studi kasus putusan mahkamah agung Nomor 131K/PDT/2010) = Juridical analysis on the defect of sale and purchase binding agreement and power of attorney and the legal implications on accusation of the heirs (Case study of the supreme court decision No 131K/PDT/2010)