Keabsahan pengikatan jual beli atas tanah yang dibuat dalam bentuk nota kesepakatan : tinjauan yuridis Putusan Kasasi Mahkamah Agung nomor 1681K/Pdt/2011 = The validity of sale and purchase agreement which made in the form of memorandum of understading : judicial review of the Supreme Court Decision number 1681K/Pdt/2011