<?xml version="1.0"?>
<dblp>
<article key="journals/tocs/Sandhu88" mdate="2006-04-03">
<author>Ravi S. Sandhu</author>
<title>The NTree: A Two Dimension Partial Order for Protection Groups.</title>
<pages>197-222</pages>
<year>1988</year>
<volume>6</volume>
<journal>ACM Trans. Comput. Syst.</journal>
<number>2</number>
<ee>http://doi.acm.org/10.1145/42186.42187</ee>
<url>db/journals/tocs/tocs6.html#Sandhu88</url>
</article>
</dblp>
