Never undo the last mount.  Make /.. work better.
 [rsc] --rw-rw-r-- M 1146153 glenda sys 33663 Dec 30 15:30 sys/src/9/port/chan.c
	/n/sourcesdump/2005/1230/plan9/sys/src/9/port/chan.c:412,418 - 
	/n/sourcesdump/2005/1231/plan9/sys/src/9/port/chan.c:412,418
	  	if(isdotdot(s)){
	  		fixdotdotname(p);
	  		DBG("addelem %s .. => rm %p\n", p->s, p->mtpt[p->mlen-1]);
	- 		if(p->mlen>0 && (c = p->mtpt[--p->mlen])){
	+ 		if(p->mlen>1 && (c = p->mtpt[--p->mlen])){
	  			p->mtpt[p->mlen] = nil;
	  			cclose(c);
	  		}