To join a layer the PLM instproc add-layer is called. This instproc
calls the PLMLayer instproc join-group which calls the PLMLayer/ns instproc join-group.
To leave a layer the PLM instproc drop-layer is called. This instproc
calls the PLMLayer instproc leave-group which calls the PLMLayer/ns instproc leave-group.
Tom Henderson
2011-11-05