/*
 *  call-seq:
 *     thgrp.enclose   => thgrp
 *  
 *  Prevents threads from being added to or removed from the receiving
 *  <code>ThreadGroup</code>. New threads can still be started in an enclosed
 *  <code>ThreadGroup</code>.
 *     
 *     ThreadGroup::Default.enclose        #=> #<ThreadGroup:0x4029d914>
 *     thr = Thread::new { Thread.stop }   #=> #<Thread:0x402a7210 sleep>
 *     tg = ThreadGroup::new               #=> #<ThreadGroup:0x402752d4>
 *     tg.add thr
 *
 *  <em>produces:</em>
 *
 *     ThreadError: can't move from the enclosed thread group
 */

static VALUE
thgroup_enclose(group)
    VALUE group;
{
    struct thgroup *data;

    Data_Get_Struct(group, struct thgroup, data);
    data->enclosed = 1;

    return group;
}