Patch [ 1213031 ] note that os.chown can have -1 as an argument

Doc/lib/libos.tex

@@ -763,7 +763,7 @@ Availability: Macintosh, \UNIX, Windows.
 
 \begin{funcdesc}{chown}{path, uid, gid}
 Change the owner and group id of \var{path} to the numeric \var{uid}
-and \var{gid}.
+and \var{gid}. To leave one of the ids unchanged, set it to -1.
 Availability: Macintosh, \UNIX.
 \end{funcdesc}